Custom pragmas for C/C++

http://lj.rossia.org/users/nashgold/71103.html

CLang actually has the full GLSL vectors support out of the box: https://clang.llvm.org/docs/LanguageExtensions.html#vectors-and-extended-vectors

GCC conservatives say they have no plans to ever support it:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88602