C99 compatibility enhancements for the configure script (#2519)

* configure.ac: Enable system extensions

Use AC_USE_SYSTEM_EXTENSIONS to build with _GNU_SOURCE (and similar
macros on non-GNU systems), to avoid an implicit declaration of the
strptime function.  This improves compatibility with future compilers
which do not support implicit function declarations.

* configure.ac: gettimeofday lives in <sys/time.h>

The gettimeofday function is declared in <sys/time.h>, not <time.h>,
according to POSIX and actual systems.  This avoids a configure
probe failure with compilers which do not support implicit function
declarations.

configure.ac

@@ -16,6 +16,7 @@ AC_PROG_CPP_WERROR
 AC_PROG_YACC
 AC_OBJEXT
 AC_EXEEXT
+AC_USE_SYSTEM_EXTENSIONS
 LT_INIT([shared static win32-dll])
 AM_PROG_CC_C_O
 
@@ -144,7 +145,7 @@ AC_FIND_FUNC([gmtime_r], [c], [#include <time.h>], [0, 0])
 AC_FIND_FUNC([gmtime], [c], [#include <time.h>], [0])
 AC_FIND_FUNC([localtime_r], [c], [#include <time.h>], [0, 0])
 AC_FIND_FUNC([localtime], [c], [#include <time.h>], [0])
-AC_FIND_FUNC([gettimeofday], [c], [#include <time.h>], [0, 0])
+AC_FIND_FUNC([gettimeofday], [c], [#include <sys/time.h>], [0, 0])
 AC_CHECK_MEMBER([struct tm.tm_gmtoff], [AC_DEFINE([HAVE_TM_TM_GMT_OFF],1,[Define to 1 if the system has the tm_gmt_off field in struct tm])],
                 [], [[#include <time.h>]])
 AC_CHECK_MEMBER([struct tm.__tm_gmtoff], [AC_DEFINE([HAVE_TM___TM_GMT_OFF],1,[Define to 1 if the system has the __tm_gmt_off field in struct tm])],