(Attempted) fix for Clang not working with MacOS

This is intended to fix #11.

Unfortunately I haven't been able to successfully build DDS yet to actually try it out, but looking at the surrounding code it seems like the right solution...

src/dds/toolchain/from_dds.cpp

         if (!compiler_id) {
             fail(context, "Cannot deduce library file extension without Compiler-ID");
         }
-        if (is_gnu) {
+        if (is_gnu_like) {
             return ".a";
         } else if (is_msvc) {
             return ".lib";
         if (!compiler_id) {
             fail(context, "Cannot deduce object file extension without Compiler-ID");
         }
-        if (is_gnu) {
+        if (is_gnu_like) {
             return ".o";
         } else if (is_msvc) {
             return ".obj";