Don't use CPP symbol _MSC_VER unless it is defined, because undefined

CPP symbols means 0.  Tiny patch from "Yang Tse" <yangsita@gmail.com>.

include/libssh2.h

@@ -63,7 +63,7 @@ extern "C" {
 # include <sys/uio.h>
 #endif
 
-#if defined(LIBSSH2_WIN32) && _MSC_VER < 1300
+#if defined(LIBSSH2_WIN32) && defined(_MSC_VER) && (_MSC_VER < 1300)
 typedef unsigned __int64 libssh2_uint64_t;
 typedef __int64 libssh2_int64_t;
 #else