we need to disable or adjust some tests where clang behaves
slightly different from GCC:
* slight difference in __FILE__ behaviour
* difference (to less useful vs GCC) in computed #include
* difference in __builtin_constant_p
* attribute(weak, alias) isn't supported by clang on MacOS (though
it could be, as Mach-O has the capabilities for this)
* the built-in assembler of clang is mediocre
this was all checked with
Apple LLVM version 10.0.1 (clang-1001.0.46.4)
9ce5b4a691