Change <cwchar> and <cstring> to look out for flags which may or may not be set by the C headers <wchar.h> and <string.h> indicating C support for the C++-altered wcschr, wcspbrk, wcsrchr, wcsstr, wmemchr, strchr, strpbrk, strrchr, memchr, and strstr.  This was already done in <cstring> for other platforms using other flags, so just had to add one more flag to the list there.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@179041 91177308-0d34-0410-b5e6-96231b3b80d8