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
diff --git a/lib/Support/Errno.cpp b/lib/Support/Errno.cpp
index 312d91e..18c6581 100644
--- a/lib/Support/Errno.cpp
+++ b/lib/Support/Errno.cpp
@@ -50,7 +50,7 @@
 # else
     strerror_r(errnum,buffer,MaxErrStrLen-1);
 # endif
-#elif defined(HAVE_STRERROR_S)  // Windows.
+#elif HAVE_DECL_STRERROR_S // "Windows Secure API"
     if (errnum)
       strerror_s(buffer, errnum);
 #elif defined(HAVE_STRERROR)