fix gmp on darwin

pkgs/development/libraries/gmp/5.1.x.nix

···
 
   nativeBuildInputs = [ m4 ];
 
+  patches = if stdenv.isDarwin then [ ./need-size-t.patch ] else null;
+
   configureFlags =
     # Build a "fat binary", with routines for several sub-architectures
     # (x86), except on Solaris where some tests crash with "Memory fault".

pkgs/development/libraries/gmp/need-size-t.patch

···
+diff --git a/gmp-h.in b/gmp-h.in
+index 7deb67a..240d663 100644
+--- a/gmp-h.in
++++ b/gmp-h.in
+@@ -46,13 +46,11 @@ along with the GNU MP Library.  If not, see http://www.gnu.org/licenses/.  */
+ #ifndef __GNU_MP__
+ #define __GNU_MP__ 5
+ 
+-#define __need_size_t  /* tell gcc stddef.h we only want size_t */
+ #if defined (__cplusplus)
+ #include <cstddef>     /* for size_t */
+ #else
+ #include <stddef.h>    /* for size_t */
+ #endif
+-#undef __need_size_t
+ 
+ /* Instantiated by configure. */
+ #if ! defined (__GMP_WITHIN_CONFIGURE)