Trac #26899: OS X Mojave without /usr/include: a range of issues

On OS X Mojave, it is possible to have Xcode installed but to have no
directory `/usr/include`.

* The configure check from #26286 finds zlib somewhere, but the Python
build does not: Python reports that it has built without zlib support.
Without zlib, pip will not build.

* gcc/gfortran needs the location of headers to be explicitly passed via
`--with-native-system-header-dir=` option. (untested)

URL: https://trac.sagemath.org/26899
Reported by: jhpalmieri
Ticket author(s): Dima Pasechnik, Volker Braun
Reviewer(s): Dima Pasechnik, Volker Braun

build/pkgs/gcc/build-gcc

@@ -18,6 +18,11 @@ GCC_CONFIGURE="$@ $GCC_CONFIGURE"
 
 # On OSX:
 if [ "$UNAME" = "Darwin" ]; then
+    # /usr/include is the latest victim in Apple's quest to get rid of standard locations
+    SDK_PATH=$(xcrun --show-sdk-path)
+    GCC_CONFIGURE="--with-sysroot=$SDK_PATH $GCC_CONFIGURE" 
+    echo "Building with XCode SDK at $SDK_PATH."
+
     # isl/cloog libraries are almost certainly from Homebrew and won't work
     GCC_CONFIGURE="--without-isl --without-cloog $GCC_CONFIGURE"
 

build/pkgs/zlib/spkg-configure.m4

@@ -5,4 +5,5 @@ SAGE_SPKG_CONFIGURE([zlib], [
         AX_CHECK_ZLIB([], [zlib_cv_libz=no])
     ])
     AS_IF([test "x$zlib_cv_libz" != "xyes"], [sage_spkg_install_zlib=yes])
+    AC_CHECK_FILE([/usr/include/],[],[sage_spkg_install_zlib=yes])
 ])