Revert "build: error on mixed declarations and code"

This reverts commit 9d6b2077.

This error is annoying while debugging, and if someone disables it for
convenience, it raises the odds of leaving ffmpeg unbuildable by default.

configure

@@ -3318,7 +3318,6 @@ 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