Add VxWorks fixincludes hack, C99 FP classification

Arrange to provide missing defs for C99 FP classification functions
and constants queried by libstdc++ configure checks (C99 support for C++98)

2021-01-10  Olivier Hainque  <hainque@adacore.com>

	* inclhack.def (vxworks_math_h_fp_c99): New hack.
	* tests/base/math.h: Update.
	* fixincl.x: Regenerate.