Some minor mingw64 porting tweaks from Glen.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@163120 91177308-0d34-0410-b5e6-96231b3b80d8

src/stdexcept.cpp

@@ -16,8 +16,13 @@
 #include <cstddef>
 #include "system_error"
 
-// Use <cxxabi.h> to determine whether we're linking against libc++abi.
-#if __has_include(<cxxabi.h>)
+#ifndef __has_include
+#define __has_include(inc) 0
+#endif
+
+#if __APPLE__
+#include <cxxabi.h>
+#elif defined(LIBCXXRT) || __has_include(<cxxabi.h>)
 #include <cxxabi.h>
 #endif