Define SDL_MAIN_HANDLED in all tests

Each test defines its own main() function. If this flag is not set, then
SDL redefines it to SDL_main(), causing compilation failures.

app/meson.build

@@ -174,6 +174,7 @@ tests = [
 foreach t : tests
     exe = executable(t[0], t[1],
                      include_directories: src_dir,
-                     dependencies: dependencies)
+                     dependencies: dependencies,
+                     c_args: ['-DSDL_MAIN_HANDLED'])
     test(t[0], exe)
 endforeach

app/tests/test_strutil.c

@@ -2,7 +2,6 @@
 #include <limits.h>
 #include <stdio.h>
 #include <string.h>
-#define SDL_MAIN_HANDLED // avoid to redefine main to SDL_main
 #include <SDL2/SDL.h>
 
 #include "util/str_util.h"