Refine graphics detection and default seelection on config

sc2/build/unix/build.config

@@ -27,6 +27,8 @@ uqm_requirements()
 
 
 	# Libraries always used
+	have_library SDL 1.2.8
+	have_library SDL2
 	have_library SDL 1.2.8 || have_library SDL2 || exit 1
 	use_library libpng || exit 1
 
@@ -206,6 +208,7 @@ uqm_prepare_config()
 	CHOICE_graphics_TITLE="Graphics Engine"
 	CHOICE_graphics_OPTION_pure_TITLE="SDL1 without OpenGL graphics support"
 	CHOICE_graphics_OPTION_pure_ACTION='graphics_pure_action'
+	CHOICE_graphics_OPTION_pure_PRECOND='have_library SDL 1.2.8'
 	graphics_pure_action() {
 		CFLAGS="$CFLAGS -DGFXMODULE_SDL -DSDL_DIR=SDL"
 		GFXMODULE=sdl
@@ -214,7 +217,7 @@ uqm_prepare_config()
 	}
 	CHOICE_graphics_OPTION_opengl_TITLE="SDL1 with OpenGL graphics support"
 	CHOICE_graphics_OPTION_opengl_ACTION='graphics_opengl_action'
-	CHOICE_graphics_OPTION_opengl_PRECOND="have_library opengl"
+	CHOICE_graphics_OPTION_opengl_PRECOND="have_library SDL 1.2.8 && have_library opengl"
 	graphics_opengl_action() {
 		CFLAGS="$CFLAGS -DGFXMODULE_SDL -DHAVE_OPENGL -DSDL_DIR=SDL"
 		GFXMODULE=sdl
@@ -223,19 +226,20 @@ uqm_prepare_config()
 	}
 	CHOICE_graphics_OPTION_sdl2_TITLE="SDL2 with modern graphics support"
 	CHOICE_graphics_OPTION_sdl2_ACTION='graphics_sdl2_action'
+	CHOICE_graphics_OPTION_sdl2_PRECOND='have_library SDL2'
 	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
+	if have_library SDL2; then
+		CHOICE_graphics_DEFAULT=sdl2
+	elif have_library opengl; then
 		CHOICE_graphics_DEFAULT=opengl
-	# fi
+	else
+		CHOICE_graphics_DEFAULT=pure
+	fi
 
 	CHOICE_sound_OPTIONS="mixsdl openal"
 	CHOICE_sound_TITLE="Sound backend"