util: Predicate the fpclassify fallback on !defined(__cplusplus)
util: Predicate the fpclassify fallback on !defined(__cplusplus)

The problem is that the fallbacks we have at the moment don't work in C++.
While we could theoretically fix the fallbacks it would also raise the
issue of correctly detecting the fpclassify function.  So, for now, we'll
just disable it until we actually have a C++ user.

Reported-by: Tom Stellard <85590917cb30a670bf23f5b4273617f8815d1e87@amd.com>
Tested-by: Tom Stellard <85590917cb30a670bf23f5b4273617f8815d1e87@amd.com>
Tested-by: EdB <bbac3597f01acc7d927683e64659598a4321d5b2@sigluy.net>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler