treewide: make -Wno-error flags clang-only

In 724e833ea2, I was a little too aggressive in enabling these flags.
Many don’t work in gcc, and we should probably avoid settings them
widely. This makes those flags optional on isclang

pkgs/tools/compression/pbzip2/default.nix

@@ -17,7 +17,7 @@ stdenv.mkDerivation rec {
 
   installFlags = "PREFIX=$(out)";
 
-  NIX_CFLAGS_COMPILE = "-Wno-error=reserved-user-defined-literal";
+  NIX_CFLAGS_COMPILE = stdenv.lib.optionalString stdenv.cc.isClang "-Wno-error=reserved-user-defined-literal";
 
   meta = with stdenv.lib; {
     homepage = http://compression.ca/pbzip2/;