Changed code slightly to avoid a warning from gcc.

--HG--
extra : convert_revision : svn%3Afdd8eb12-d10e-0410-9acb-85c331704f74/trunk%403758

dlib/svm/kernel_matrix.h

@@ -79,7 +79,13 @@ namespace dlib
         {}
 
         template <typename T>
+        // This funny #ifdef thing is here because gcc sometimes gives a warning 
+        // about v being unused otherwise.
+#ifdef ENABLE_ASSERTS
         void assert_is_vector(const matrix_exp<T>& v)
+#else
+        void assert_is_vector(const matrix_exp<T>& )
+#endif
         {
             // make sure requires clause is not broken
             DLIB_ASSERT(is_vector(v) == true,