fix static assertion problem for gcc (bug 4022)

src/mesa/shader/slang/slang_utility.h

@@ -29,7 +29,10 @@
 extern "C" {
 #endif
 
-#define static_assert(expr) do { int _array[(expr) ? 1 : 0]; _array[0]; } while (0)
+/* Compile-time assertions.  If the expression is zero, try to declare an
+ * array of size [-1] to cause compilation error.
+ */
+#define static_assert(expr) do { int _array[(expr) ? 1 : -1]; _array[0]; } while (0)
 
 void slang_alloc_free (void *);
 void *slang_alloc_malloc (unsigned int);