Changed Windows 64bit OS define to x86_64.

Also added check for __x86_64__ define since MinGW64 seems to define
the _M_X64 macro through a header not available for config-win32.h.

lib/config-win32.h

@@ -653,10 +653,10 @@
 #undef OS
 #if defined(_M_IX86) || defined(__i386__) /* x86 (MSVC or gcc) */
 #define OS "i386-pc-win32"
+#elif defined(_M_X64) || defined(__x86_64__) /* x86_64 (MSVC >=2005 or gcc) */
+#define OS "x86_64-pc-win32"
 #elif defined(_M_IA64) /* Itanium */
 #define OS "ia64-pc-win32"
-#elif defined(_M_X64) /* AMD64/EM64T - Not defined until MSVC 2005 */
-#define OS "amd64-pc-win32"
 #else
 #define OS "unknown-pc-win32"
 #endif