backends: add XASH_MESSAGEBOX to specify messagebox platform implementation

common/backends.h

@@ -47,4 +47,10 @@ GNU General Public License for more details.
 #define TIMER_LINUX 2
 #define TIMER_WIN32 3
 
+// messageboxes (XASH_MESSAGEBOX)
+#define MSGBOX_STDERR 0
+#define MSGBOX_SDL 1
+#define MSGBOX_ANDROID 2
+#define MSGBOX_WIN32 3
+
 #endif /* BACKENDS_H */

common/defaults.h

@@ -31,10 +31,18 @@ SETUP BACKENDS DEFINITIONS
 		#ifndef XASH_TIMER
 			#define XASH_TIMER TIMER_SDL
 		#endif
+
+		#ifndef XASH_MESSAGEBOX
+			#define XASH_MESSAGEBOX MSGBOX_SDL
+		#endif
 	#else
 		#ifndef XASH_TIMER
 			#define XASH_TIMER TIMER_LINUX
 		#endif
+
+		#ifndef XASH_MESSAGEBOX
+			#define XASH_MESSAGEBOX MSGBOX_STDERR
+		#endif
 	#endif
 
 	#ifdef XASH_SDL
@@ -70,6 +78,10 @@ SETUP BACKENDS DEFINITIONS
 		#ifndef XASH_SOUND
 			#define XASH_SOUND SOUND_OPENSLES
 		#endif
+
+		#ifndef XASH_MESSAGEBOX
+			#define XASH_MESSAGEBOX MSGBOX_ANDROID
+		#endif
 	#endif // android case
 
 	#ifdef XASH_FBDEV
@@ -88,6 +100,10 @@ SETUP BACKENDS DEFINITIONS
 		#ifndef XASH_SOUND
 			#define XASH_SOUND SOUND_ALSA
 		#endif
+
+		#ifndef XASH_MESSAGEBOX
+			#define XASH_MESSAGEBOX MSGBOX_STDERR
+		#endif
 		#define XASH_USE_EVDEV
 	#endif // android case
 
@@ -130,6 +146,10 @@ SETUP BACKENDS DEFINITIONS
 	#define XASH_INPUT INPUT_NULL
 #endif
 
+#ifndef XASH_MESSAGEBOX
+	#define XASH_MESSAGEBOX MSGBOX_STDERR
+#endif
+
 /*
 =========================================================================