lib/Support/Errno.cpp: Check strerror_s() with HAVE_DECL_STRERROR_S in config.h.*.

lib/Support/Errno.cpp: Check strerror_s() with HAVE_DECL_STRERROR_S in config.h.*.

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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@125172 91177308-0d34-0410-b5e6-96231b3b80d8