Fix #12138 - use cc on OpenBSD and link libm

[euantorano]

Fixes #12138.

This patch uses cc rather than gcc on openBSD (the GCC shipped is an old version - gcc version 4.2.1 20070719 as of OpenBSD 6.7; cc is clang OpenBSD clang version 8.0.1 (tags/RELEASE_801/final) (based on LLVM 8.0.1)).

It also links libm as mentioned in #12138 (comment).

The detect tool now builds and runs successfully on OpenBSD:

bin/nim c -r tools/detect/detect.nim
Hint: used config file '/home/euan/src/Nim/config/nim.cfg' [Conf]
Hint: used config file '/home/euan/src/Nim/config/config.nims' [Conf]
..................
/home/euan/src/Nim/tools/detect/detect.nim(46, 3) Hint: 'ifile' is declared but not used [XDeclaredButNotUsed]
/home/euan/src/Nim/tools/detect/detect.nim(20, 5) Hint: 'cpp' is declared but not used [XDeclaredButNotUsed]
Hint: clang -c  -w   -I/home/euan/src/Nim/lib -I/home/euan/src/Nim/tools/detect -o /home/euan/.cache/nim/detect_d/@mdetect.nim.c.o /home/euan/.cache/nim/detect_d/@mdetect.nim.c [Exec]
Hint:  [Link]
Hint: 61035 lines; 37.213s; 98.68MiB peakmem; Debug build; proj: /home/euan/src/Nim/tools/detect/detect.nim; out: /home/euan/src/Nim/tools/detect/detect [SuccessX]
Hint: /home/euan/src/Nim/tools/detect/detect  [Exec]
CMD cc -o testh testh.c
testh.c:1:10: fatal error: 'aio.h' file not found
#include <aio.h>
         ^~~~~~~
1 error generated.
Not found: <aio.h>
CMD cc -o testh testh.c
Found: <dlfcn.h>
CMD cc -o testh testh.c
Found: <errno.h>
CMD cc -o testh testh.c
Found: <fcntl.h>
CMD cc -o testh testh.c
Found: <fenv.h>
CMD cc -o testh testh.c
testh.c:1:10: fatal error: 'fmtmsg.h' file not found
#include <fmtmsg.h>
         ^~~~~~~~~~
1 error generated.
Not found: <fmtmsg.h>
CMD cc -o testh testh.c
Found: <fnmatch.h>
CMD cc -o testh testh.c
Found: <ftw.h>
CMD cc -o testh testh.c
Found: <glob.h>
CMD cc -o testh testh.c
Found: <langinfo.h>
CMD cc -o testh testh.c
Found: <locale.h>
CMD cc -o testh testh.c
Found: <netdb.h>
CMD cc -o testh testh.c
Found: <net/if.h>
CMD cc -o testh testh.c
Found: <netinet/in.h>
CMD cc -o testh testh.c
In file included from testh.c:1:
/usr/include/netinet/tcp.h:42:9: error: unknown type name 'u_int32_t'; did you mean '__int128_t'?
typedef u_int32_t tcp_seq;
        ^
note: '__int128_t' declared here
/usr/include/netinet/tcp.h:49:2: error: unknown type name 'u_int16_t'; did you mean '__int128_t'?
        u_int16_t th_sport;             /* source port */
        ^
note: '__int128_t' declared here
/usr/include/netinet/tcp.h:50:2: error: unknown type name 'u_int16_t'; did you mean '__int128_t'?
        u_int16_t th_dport;             /* destination port */
        ^
note: '__int128_t' declared here
/usr/include/netinet/tcp.h:54:2: error: unknown type name 'u_int32_t'; did you mean '__int128_t'?
        u_int32_t th_x2:4,              /* (unused) */
        ^
note: '__int128_t' declared here
/usr/include/netinet/tcp.h:58:2: error: unknown type name 'u_int32_t'; did you mean '__int128_t'?
        u_int32_t th_off:4,             /* data offset */
        ^
note: '__int128_t' declared here
/usr/include/netinet/tcp.h:58:12: error: duplicate member 'th_off'
        u_int32_t th_off:4,             /* data offset */
                  ^
/usr/include/netinet/tcp.h:55:5: note: previous declaration is here
                  th_off:4;             /* data offset */
                  ^
/usr/include/netinet/tcp.h:59:5: error: duplicate member 'th_x2'
                  th_x2:4;              /* (unused) */
                  ^
/usr/include/netinet/tcp.h:54:12: note: previous declaration is here
        u_int32_t th_x2:4,              /* (unused) */
                  ^
/usr/include/netinet/tcp.h:61:2: error: unknown type name 'u_int8_t'
        u_int8_t  th_flags;
        ^
/usr/include/netinet/tcp.h:70:2: error: unknown type name 'u_int16_t'; did you mean '__int128_t'?
        u_int16_t th_win;                       /* window */
        ^
note: '__int128_t' declared here
/usr/include/netinet/tcp.h:71:2: error: unknown type name 'u_int16_t'; did you mean '__int128_t'?
        u_int16_t th_sum;                       /* checksum */
        ^
note: '__int128_t' declared here
/usr/include/netinet/tcp.h:72:2: error: unknown type name 'u_int16_t'; did you mean '__int128_t'?
        u_int16_t th_urp;                       /* urgent pointer */
        ^
note: '__int128_t' declared here
11 errors generated.
Not found: <netinet/tcp.h>
CMD cc -o testh testh.c
Found: <nl_types.h>
CMD cc -o testh testh.c
Found: <poll.h>
CMD cc -o testh testh.c
Found: <pthread.h>
CMD cc -o testh testh.c
Found: <sched.h>
CMD cc -o testh testh.c
Found: <semaphore.h>
CMD cc -o testh testh.c
Found: <signal.h>
CMD cc -o testh testh.c
Found: <sys/ipc.h>
CMD cc -o testh testh.c
Found: <sys/mman.h>
CMD cc -o testh testh.c
Found: <sys/resource.h>
CMD cc -o testh testh.c
Found: <sys/select.h>
CMD cc -o testh testh.c
Found: <sys/socket.h>
CMD cc -o testh testh.c
Found: <sys/stat.h>
CMD cc -o testh testh.c
Found: <sys/statvfs.h>
CMD cc -o testh testh.c
Found: <sys/wait.h>
CMD cc -o testh testh.c
Found: <spawn.h>
CMD cc -o testh testh.c
Found: <stdio.h>
CMD cc -o testh testh.c
Found: <time.h>
CMD cc -o testh testh.c
Found: <unistd.h>
CMD cc -lm -o genconsts genconsts.c
genconsts.c:444:48: warning: format specifies type 'int' but the argument has type 'const fenv_t *' [-Wformat]
  fprintf(f, "const FE_DFL_ENV* = cint(%d)\n", FE_DFL_ENV);
                                       ~~      ^~~~~~~~~~
/usr/include/machine/fenv.h:99:21: note: expanded from macro 'FE_DFL_ENV'
#define FE_DFL_ENV              ((const fenv_t *)&__fe_dfl_env)
                                ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
genconsts.c:1121:57: warning: format specifies type 'int' but the argument has type 'void (*)(int)' [-Wformat]
  fprintf(f, "const SIG_DFL* = cast[Sighandler](%d)\n", SIG_DFL);
                                                ~~      ^~~~~~~
/usr/include/sys/signal.h:97:18: note: expanded from macro 'SIG_DFL'
#define SIG_DFL         (void (*)(int))0
                        ^~~~~~~~~~~~~~~~
genconsts.c:1124:57: warning: format specifies type 'int' but the argument has type 'void (*)(int)' [-Wformat]
  fprintf(f, "const SIG_ERR* = cast[Sighandler](%d)\n", SIG_ERR);
                                                ~~      ^~~~~~~
/usr/include/sys/signal.h:99:18: note: expanded from macro 'SIG_ERR'
#define SIG_ERR         (void (*)(int))-1
                        ^~~~~~~~~~~~~~~~~
genconsts.c:1127:57: warning: format specifies type 'int' but the argument has type 'void (*)(int)' [-Wformat]
  fprintf(f, "const SIG_IGN* = cast[Sighandler](%d)\n", SIG_IGN);
                                                ~~      ^~~~~~~
/usr/include/sys/signal.h:98:18: note: expanded from macro 'SIG_IGN'
#define SIG_IGN         (void (*)(int))1
                        ^~~~~~~~~~~~~~~~
genconsts.c:1140:49: warning: format specifies type 'int' but the argument has type 'key_t' (aka 'long') [-Wformat]
  fprintf(f, "const IPC_PRIVATE* = cint(%d)\n", IPC_PRIVATE);
                                        ~~      ^~~~~~~~~~~
                                        %ld
/usr/include/sys/ipc.h:73:21: note: expanded from macro 'IPC_PRIVATE'
#define IPC_PRIVATE     (key_t)0 /* private key */
                        ^~~~~~~~
genconsts.c:1428:47: warning: format specifies type 'int' but the argument has type 'unsigned long' [-Wformat]
  fprintf(f, "const ST_RDONLY* = cint(%d)\n", ST_RDONLY);
                                      ~~      ^~~~~~~~~
                                      %lu
/usr/include/sys/statvfs.h:38:19: note: expanded from macro 'ST_RDONLY'
#define ST_RDONLY       0x0001UL        /* read-only filesystem */
                        ^~~~~~~~
genconsts.c:1431:47: warning: format specifies type 'int' but the argument has type 'unsigned long' [-Wformat]
  fprintf(f, "const ST_NOSUID* = cint(%d)\n", ST_NOSUID);
                                      ~~      ^~~~~~~~~
                                      %lu
/usr/include/sys/statvfs.h:39:19: note: expanded from macro 'ST_NOSUID'
#define ST_NOSUID       0x0002UL        /* nosuid flag set */
                        ^~~~~~~~
genconsts.c:1492:54: warning: format specifies type 'long' but the argument has type 'int' [-Wformat]
  fprintf(f, "const CLOCKS_PER_SEC* = clong(%ld)\n", CLOCKS_PER_SEC);
                                            ~~~      ^~~~~~~~~~~~~~
                                            %d
/usr/include/time.h:71:24: note: expanded from macro 'CLOCKS_PER_SEC'
#define CLOCKS_PER_SEC  100     /* frequency of ticks reported by clock().  */
                        ^~~
8 warnings generated.
CMD cc -lm -E -o pre.i pre.c
cc: warning: -lm: 'linker' input unused [-Wunused-command-line-argument]
CMD ./genconsts
Success