build: Use macOS' compatibility macros to enable all features.

* configure.ac: On macOS, use the compatibility macros to expose every
feature of the libc.  This is the equivalent of _GNU_SOURCE on GNU
libc.
--
Not defining this leads to compilation errors or superfluous warnings
on macOS.

GnuPG-bug-id: 2910
Signed-off-by: Justus Winter <justus@g10code.com>

configure.ac

@@ -712,6 +712,10 @@ case "${host}" in
         # keeps things simple
         require_iconv=no
         ;;
+    *-apple-darwin*)
+        AC_DEFINE(_DARWIN_C_SOURCE, 900000L,
+                  Expose all libc features (__DARWIN_C_FULL).)
+        ;;
     *)
        ;;
 esac