Refine fenv.h handling: check if the desired macros exist, before using
it. NetBSD/ARM and TILE-Gx are examples for platforms that have an
unusable fenv.h and this avoids the need for a blacklist.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@177865 91177308-0d34-0410-b5e6-96231b3b80d8