graphite2: pull in libgcc if not using gcc for compilation

graphite2 needs to link against libgcc. If stdenv.cc.isGNU this always
works, but for clang based stdenvs, the build would fail. We can pull in
the standalone libgcc derivation in those cases.

pkgs/development/libraries/silgraphite/graphite2.nix

@@ -5,6 +5,7 @@
 , freetype
 , cmake
 , static ? stdenv.hostPlatform.isStatic
+, libgcc
 }:
 
 stdenv.mkDerivation rec {
@@ -18,7 +19,8 @@ stdenv.mkDerivation rec {
   };
 
   nativeBuildInputs = [ pkg-config cmake ];
-  buildInputs = [ freetype ];
+  buildInputs = [ freetype ]
+    ++ lib.optionals (!stdenv.cc.isGNU) [ libgcc ];
 
   patches = lib.optionals stdenv.isDarwin [ ./macosx.patch ];