Only suppress warning for versions of GCC where it exists.

Author: brettle

src/webots/maths/WbMathsUtilities.cpp

@@ -109,7 +109,9 @@ int WbMathsUtilities::convexHull(const QVector<WbVector2> &points, QVector<int>
 
   // Ignore false positive warning produced by some versions of GCC.
 #pragma GCC diagnostic push
+#if __GNUC__ >= 7
 #pragma GCC diagnostic ignored "-Wstringop-overflow"
+#endif
   QVector<int> index(size);
 #pragma GCC diagnostic pop
   for (i = 0; i < size; ++i)