Enhance config script to allow selecting SDL1 or 2 backends

sc2/build/unix/build.config

@@ -27,7 +27,7 @@ uqm_requirements()
 
 
 	# Libraries always used
-	use_library SDL 1.2.8 || exit 1
+	have_library SDL 1.2.8 || have_library SDL2 || exit 1
 	use_library libpng || exit 1
 
 	case "$HOST_SYSTEM" in
@@ -194,25 +194,40 @@ uqm_prepare_config()
 	esac
 	
 
-	CHOICE_graphics_OPTIONS="pure opengl"
-	CHOICE_graphics_TITLE="OpenGL graphics support"
-	CHOICE_graphics_OPTION_pure_TITLE="Don't include OpenGL graphics support"
+	CHOICE_graphics_OPTIONS="pure opengl sdl2"
+	CHOICE_graphics_TITLE="Graphics Engine"
+	CHOICE_graphics_OPTION_pure_TITLE="SDL1 without OpenGL graphics support"
 	CHOICE_graphics_OPTION_pure_ACTION='graphics_pure_action'
 	graphics_pure_action() {
-		CFLAGS="$CFLAGS -DGFXMODULE_SDL"
+		CFLAGS="$CFLAGS -DGFXMODULE_SDL -DSDL_DIR=SDL"
 		GFXMODULE=sdl
 		HAVE_OPENGL=0
+		use_library SDL 1.2.8
 	}
-	CHOICE_graphics_OPTION_opengl_TITLE="Include OpenGL graphics support"
+	CHOICE_graphics_OPTION_opengl_TITLE="SDL1 with OpenGL graphics support"
 	CHOICE_graphics_OPTION_opengl_ACTION='graphics_opengl_action'
 	CHOICE_graphics_OPTION_opengl_PRECOND="have_library opengl"
 	graphics_opengl_action() {
-		CFLAGS="$CFLAGS -DGFXMODULE_SDL -DHAVE_OPENGL"
+		CFLAGS="$CFLAGS -DGFXMODULE_SDL -DHAVE_OPENGL -DSDL_DIR=SDL"
 		GFXMODULE=sdl
 		HAVE_OPENGL=1
-		use_library opengl
+		use_library SDL 1.2.8 && use_library opengl
 	}
+	CHOICE_graphics_OPTION_sdl2_TITLE="SDL2 with modern graphics support"
+	CHOICE_graphics_OPTION_sdl2_ACTION='graphics_sdl2_action'
+	graphics_sdl2_action() {
+		CFLAGS="$CFLAGS -DGFXMODULE_SDL -DSDL_DIR=SDL2"
+		GFXMODULE=sdl
+		HAVE_OPENGL=0
+		use_library SDL2
+	}
+	# TODO: Once SDL2 has parity to SDL1, make it the default if
+	#       available.
+	# if have_library SDL2; then
+	#	CHOICE_graphics_DEFAULT=sdl2
+	# else
 	CHOICE_graphics_DEFAULT=opengl
+	# fi
 
 	CHOICE_sound_OPTIONS="mixsdl openal"
 	CHOICE_sound_TITLE="Sound backend"