AC_CHECK_FUNCS seeks a symbol only in libs. We should check the declaration in string.h.

FIXME: I have never seen mingw(s) have strerror_s() (not _strerror_s()).
FIXME: Autoconf/CMake may seek strerror_s() with the definition MINGW_HAS_SECURE_API in future.
llvm-svn: 125172