gimpPlugins.fourier: fix build on clang

pkgs/applications/graphics/gimp/plugins/default.nix

···
     buildInputs = with pkgs; [ fftw ];
 
     postPatch = ''
+      substituteInPlace Makefile --replace '$(GCC)' '$(CC)'
+
       # The tarball contains a prebuilt binary.
       make clean
     '';