libxmi: fix darwin build

Library requires C std 89 to compile. Clang defaults to newer these
days.

pkgs/development/libraries/libxmi/default.nix

@@ -9,6 +9,8 @@ stdenv.mkDerivation rec {
     sha256 = "03d4ikh29l38rl1wavb0icw7m5pp7yilnv7bb2k8qij1dinsymlx";
   };
 
+  CFLAGS = "-std=gnu89";
+
   # For the x86_64 GNU/Linux arch to be recognized by 'configure'
   preConfigure = "cp ${libtool}/share/libtool/build-aux/config.sub .";