fix compiler warning: zero used for undefined preprocessing identifier

include/curl/curl.h

@@ -1779,7 +1779,8 @@ CURL_EXTERN CURLcode curl_easy_pause(CURL *handle, int bitmask);
 #include "multi.h"
 
 /* the typechecker doesn't work in C++ (yet) */
-#if (__GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 3)) && \
+#if defined(__GNUC__) && defined(__GNUC_MINOR__) && \
+    ((__GNUC__ > 4) || (__GNUC__ == 4 && __GNUC_MINOR__ >= 3)) && \
     !defined(__cplusplus) && !defined(CURL_DISABLE_TYPECHECK)
 #include "typecheck-gcc.h"
 #else