Build: Use -Wvla with GCC if supported.

Variable-length arrays are mandatory in C99 but optional in C11.
The code doesn't currently use any VLAs and it shouldn't in the
future either to stay compatible with C11 without requiring any
optional C11 features.

configure.ac

@@ -560,6 +560,7 @@ if test "$GCC" = yes ; then
 	for NEW_FLAG in \
 			-Wall \
 			-Wextra \
+			-Wvla \
 			-Wformat=2 \
 			-Winit-self \
 			-Wmissing-include-dirs \