build: error on variable-length arrays

With all the VLAs gone, make sure they never come back.
Signed-off-by: Mans Rullgard <mans@mansr.com>

configure

@@ -3504,6 +3504,7 @@ elif enabled gcc; then
     check_cflags -Werror=implicit-function-declaration
     check_cflags -Werror=missing-prototypes
     check_cflags -Werror=declaration-after-statement
+    check_cflags -Werror=vla
 elif enabled llvm_gcc; then
     check_cflags -mllvm -stack-alignment=16
 elif enabled clang; then