cmath: Use c99 math on a new enough msvcrt

MSVCRT 14+ supports the C99 math routines that we need.  Use them
accordingly.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@295509 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed