Don't attempt to link libstdc++fs when using Clang

On MacOS and (I think) most BSDs the default standard library is libc++, and so attempting to link with libstdc++fs does not work.

On Linux, where (AFAIK) most distros use Clang/libstdc++, this is going to mean link-time failures when using std::filesystem. In this case the user can add Link-Flags: -lstdc++fs to their toolchain file themselves as a fix.

src/dds/toolchain/from_dds.cpp

         string_seq ret;
         if (is_msvc) {
             ret = {get_compiler(language::cxx), "/nologo", "/EHsc", "<IN>", "/Fe<OUT>"};
-        } else if (is_gnu_like) {
+        } else if (is_gnu) {
             ret = {get_compiler(language::cxx),
                    "-fPIC",
                    "-fdiagnostics-color",
                    "-pthread",
                    "-lstdc++fs",
                    "-o<OUT>"};
+        } else if (is_clang) {
+            ret = {get_compiler(language::cxx),
+                   "-fPIC",
+                   "-fdiagnostics-color",
+                   "<IN>",
+                   "-pthread",
+                   "-o<OUT>"};
         } else {
             assert(false && "No link-exe command");
             std::terminate();