build: error on mixed declarations and code

Signed-off-by: Mans Rullgard <mans@mansr.com>

configure

@@ -3118,6 +3118,7 @@ elif enabled gcc; then
     check_cflags -fno-tree-vectorize
     check_cflags -Werror=implicit-function-declaration
     check_cflags -Werror=missing-prototypes
+    check_cflags -Werror=declaration-after-statement
 elif enabled llvm_gcc; then
     check_cflags -mllvm -stack-alignment=16
 elif enabled clang; then