Better compiler vendor detection

[devinamatthews]

The autoconf macro ax_compiler_vendor provides an arguably better way to determine the compiler vendor, since it doesn't depend on the vagaries of compiler version strings (although it does require running the compiler). A similar technique could be implemented in configure even though it's not autoconf-based.

Credit to @marcinz for the suggestion via https://github.com/devinamatthews/tblis/issues/46.