Allow -Wno- flags to pass through

These inhibit warnings-as-errors, so allow them to pass through.

Library/ENV/4.3/cc

@@ -132,7 +132,7 @@ class Cmd
         # clang doesn't support OpenMP
         args << arg if not tool =~ /^clang/
       when /^-W.*/
-        args << arg if arg =~ /^-W[alp],/
+        args << arg if arg =~ /^-W[alp],/ or arg =~ /^-Wno-/
       when '-macosx_version_min', '-dylib_install_name'
         args << "-Wl,#{arg},#{whittler.next}"
       when /^-isysroot/