[SDL2] On Win, build and link statically.

Keep using system lib dynamically on Unix.

third_party/SDL2.lua

@@ -0,0 +1,36 @@
+--
+-- On Linux we build against the system version (libsdl2-dev for building),
+-- since SDL2 is our robust API there like DirectX is on Windows.
+--
+
+local sdl2_sys_includedirs = {}
+local third_party_path = os.getcwd()
+
+if os.istarget("windows") then
+  -- build ourselves
+  include("SDL2-static.lua")
+else
+  -- use system libraries
+  local result, code, what = os.outputof("sdl2-config --cflags")
+  if result then
+    for inc in string.gmatch(result, "-I([%S]+)") do
+      table.insert(sdl2_sys_includedirs, inc)
+    end
+  else
+    error("Failed to run 'sdl2-config'. Are libsdl2 development files installed?")
+  end
+end
+
+
+--
+-- Call this function in project scope to include the SDL2 headers.
+--
+function sdl2_include()
+  filter("platforms:Windows")
+    includedirs({
+      path.getrelative(".", third_party_path) .. "/SDL2/include",
+    })
+  filter("platforms:Linux")
+    includedirs(sdl2_sys_includedirs)
+  filter({})
+end