Cannot compile with older gcc / clang

[ronaaron]

I'm supporting older compiler versions (clang 5, gcc 4) for my toolset on some platforms (Windows, RPI) and encounter compiler failures with the newest (60d76b7):

clang:

error in backend: Cannot select: intrinsic %llvm.thread.pointer

gcc (mingw):

error: missing binary operator before token "("
 #elif defined(__clang__) && __has_builtin(__builtin_assume)