third_party:build: Test for the flags, recognized by Clang.

This is the most complicated, but working solution to this simple problem.

The easiest one should be to rename file to aesni-intel_asm.S(note capital S), which it is actually is. I guess the only reason it's not called so is the absence of the appropriate handling in WAF?

Other option is to use -xassembler-with-cpp flag for gcc as well, as it's it older, but recognized option.

And, most (relatively) complicated is this patch.