Only use __int64 on MSVC, not on all win32 compilers

Mingw32 doesn't define the __int64 types.

This was implicitly reverted in AOSP (where this change was
already merged before) by the new code drop from Fraunhofer.

libSYS/include/machine_type.h

@@ -173,7 +173,7 @@ amm-info@iis.fraunhofer.de
 
 
 /* Define 64 bit base integer type. */
-#ifdef _WIN32
+#ifdef _MSC_VER
   typedef __int64 INT64;
   typedef unsigned __int64 UINT64;
 #else