sdl12-compat: add -Wall to CFLAGS, too, for gcc/clang.

SDL Commits
1 
From 4a04f9230781fe871790392e6d34fe7498be4878 Mon Sep 17 00:00:00 2001
From: Ozkan Sezer <[EMAIL REDACTED]>
Date: Wed, 2 Jun 2021 05:10:02 +0300
Subject: [PATCH] add -Wall to CFLAGS, too, for gcc/clang.

---
 CMakeLists.txt | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/CMakeLists.txt b/CMakeLists.txt
index f73ac90..f5ef689 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -38,7 +38,7 @@ include("cmake/modules/FindSDL2.cmake")
 target_include_directories(SDL PRIVATE ${SDL2_INCLUDE_DIRS})
 
 if (CMAKE_C_COMPILER_ID MATCHES "Clang|GNU")
-  add_definitions("-Werror=declaration-after-statement")  # force this universally.
+  add_definitions("-Wall -Werror=declaration-after-statement")  # force these universally.
 endif()
 
 # avoid DLL having 'lib' prefix with Windows MinGW builds