SDL: cmake: Don't use the CFLAGS environment variable.

From f79ac6a8de2902bf9cdd733f1857485133623ab8 Mon Sep 17 00:00:00 2001
From: "Ryan C. Gordon" <[EMAIL REDACTED]>
Date: Mon, 27 Sep 2021 23:31:23 -0400
Subject: [PATCH] cmake: Don't use the CFLAGS environment variable.

CMake has standard means of setting compiler flags, such as the
-DCMAKE_C_FLAGS command line for general-purpose flags, and
-DCMAKE_BUILD_TYPE for letting it choose reasonable release/debug/etc
defaults. Trying to emulate the configure script is incorrect and confusing
here.

Fixes #1819.
---
 CMakeLists.txt | 15 ++-------------
 1 file changed, 2 insertions(+), 13 deletions(-)

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 0a22a556b4..7ecaf43ba1 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -212,19 +212,8 @@ if(UNIX OR MINGW OR MSYS OR (USE_CLANG AND NOT WINDOWS) OR VITA)
   set(OPT_DEF_LIBC ON)
 endif()
 
-# Default flags, if not set otherwise
-if("$ENV{CFLAGS}" STREQUAL "")
-  if(CMAKE_BUILD_TYPE STREQUAL "")
-    if(USE_GCC OR USE_CLANG)
-      set(CMAKE_C_FLAGS "-g -O3")
-    endif()
-  endif()
-else()
-  set(CMAKE_C_FLAGS "$ENV{CFLAGS}")
-  list(APPEND EXTRA_CFLAGS "$ENV{CFLAGS}")
-endif()
-if(NOT ("$ENV{CFLAGS}" STREQUAL "")) # Hackish, but does the trick on Win32
-  list(APPEND EXTRA_LDFLAGS "$ENV{LDFLAGS}")
+if(NOT ("$ENV{CFLAGS}" STREQUAL ""))
+  message(WARNING "SDL's CMakeLists.txt no longer checks the CFLAGS environment. Please use CMake's CMAKE_C_FLAGS and CMAKE_BUILD_TYPE variables directly.")
 endif()
 
 if(MSVC)