Do not define Darwin SDL entrypoints if SDL is disabled

The SDL related initialization functions should only be compiled in if SDL has also been enabled in the build. This was discovered after getting build errors on macOS with SDL disabled.

src/conio/ciolib.c

@@ -1886,7 +1886,7 @@ CIOLIBEXPORT enum ciolib_codepage ciolib_getcodepage(void)
 	return conio_fontdata[font].cp;
 }
 
-#if defined(__DARWIN__)
+#if defined(__DARWIN__) && defined(WITH_SDL)
 #ifdef main
 #undef main
 #endif