Use GNUC macro only to check for is_trivially_copyable support

[Manu343726]

Basically, don't assume builds using clang could not use the GNU stdlibc++ standard library. Found when setting up the CI build to report standardese/cppast#33 :)

Here's a type_safe_stdlibcxx_istriviallycopyable_trait branch CI build that reproduces the problem (As with cppast 33, using the geordiejones/cpp14-build-machine docker image to build cppast with Clang 5.0).

[foonathan]

Yes, thank you.

[Manu343726]

Maybe you could update the cppast dependency to integrate this fix? Thanks :)