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/development/libraries/glib/default.nix

@@ -7,7 +7,7 @@
 
 with stdenv.lib;
 
-assert stdenv.gcc.gcc != null;
+assert !stdenv.isDarwin -> stdenv.cc ? gcc;
 
 # TODO:
 # * Add gio-module-fam
@@ -83,7 +83,7 @@ stdenv.mkDerivation rec {
   preCheck = optionalString doCheck
     # libgcc_s.so.1 must be installed for pthread_cancel to work
     # also point to the glib/.libs path
-    '' export LD_LIBRARY_PATH="${stdenv.gcc.gcc}/lib:$NIX_BUILD_TOP/${name}/glib/.libs:$LD_LIBRARY_PATH"
+    '' export LD_LIBRARY_PATH="${stdenv.cc.gcc}/lib:$NIX_BUILD_TOP/${name}/glib/.libs:$LD_LIBRARY_PATH"
        export TZDIR="${tzdata}/share/zoneinfo"
        export XDG_CACHE_HOME="$TMP"
        export XDG_RUNTIME_HOME="$TMP"