Add extsources to libsdl_ttf (#568)

packages/l/libsdl_ttf/xmake.lua

@@ -11,6 +11,12 @@ package("libsdl_ttf")
         add_versions("2.0.15", "cdb72b5b1c3b27795fa128af36f369fee5d3e38a96c350855da0b81880555dbc")
     end
 
+    if is_plat("linux") then
+        add_extsources("pacman::sdl2_ttf", "apt::libsdl2-ttf-dev")
+    elseif is_plat("macosx") then
+        add_extsources("brew::sdl2_ttf")
+    end
+
     add_deps("libsdl")
     if is_plat("linux", "macosx") then
         add_deps("freetype")