Change occurrences of gcc to the more general cc

This is done for the sake of Yosemite, which does not have gcc, and yet
this change is also compatible with Linux.

pkgs/os-specific/linux/opengl/xorg-sys/default.nix

@@ -11,6 +11,6 @@
 stdenv.mkDerivation {
   name = "xorg-sys-opengl-3";
   builder = ./builder.sh;
-  neededLibs = [xlibs.libXxf86vm xlibs.libXext expat libdrm stdenv.gcc.gcc];
+  neededLibs = [xlibs.libXxf86vm xlibs.libXext expat libdrm stdenv.cc.gcc];
 }