[client] ds: make SDL display server optional

This commit also moved the SDL dependencies into the SDL backend so that
-DENABLE_SDL=NO builds do not link against SDL.

client/displayservers/CMakeLists.txt

@@ -28,7 +28,9 @@ if (ENABLE_X11)
 endif()
 
 # SDL must be last as it's the fallback implemntation
-add_displayserver(SDL)
+if (ENABLE_SDL)
+  add_displayserver(SDL)
+endif()
 
 list(REMOVE_AT DISPLAYSERVERS      0)
 list(REMOVE_AT DISPLAYSERVERS_LINK 0)

client/displayservers/SDL/CMakeLists.txt

@@ -1,10 +1,10 @@
 cmake_minimum_required(VERSION 3.0)
 project(displayserver_SDL LANGUAGES C)
 
-#find_package(PkgConfig)
-#pkg_check_modules(DISPLAYSERVER_SDL_PKGCONFIG REQUIRED
-#	#sdl2
-#)
+find_package(PkgConfig)
+pkg_check_modules(DISPLAYSERVER_SDL_PKGCONFIG REQUIRED
+	sdl2
+)
 
 add_library(displayserver_SDL STATIC
 	sdl.c