sdl12-compat: cmake: set SDLMain.m source language as C for older cmake.

SDL Commits
1 
From 51254e57b346666e2835df79823110b2a9acfb1a Mon Sep 17 00:00:00 2001
From: Ozkan Sezer <[EMAIL REDACTED]>
Date: Fri, 9 Jul 2021 11:55:10 +0300
Subject: [PATCH] cmake: set SDLMain.m source language as C for older cmake.

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

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 905e5c5..14ea7d3 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -126,6 +126,7 @@ endif()
 # SDLmain library...
 if(APPLE)
     add_library(SDLmain STATIC src/SDLmain/macosx/SDLMain.m)
+    set_source_files_properties(src/SDLmain/macosx/SDLMain.m PROPERTIES LANGUAGE C)
 elseif(WIN32)
     add_library(SDLmain STATIC src/SDLmain/win32/SDL_win32_main.c)
     set_target_properties(SDLmain PROPERTIES COMPILE_DEFINITIONS "_CRT_SECURE_NO_WARNINGS;_CRT_SECURE_NO_DEPRECATE;_CRT_NONSTDC_NO_DEPRECATE")  # !!! FIXME: don't use C runtime? We fixed this in SDL2.