Added warning when SDL2_DIR env var is not set on Windows.

scripts/genie.lua

@@ -49,6 +49,14 @@ toolchain(BGFX_BUILD_DIR, BGFX_THIRD_PARTY_DIR)
 function copyLib()
 end
 
+if _OPTIONS["with-sdl"] then
+	if os.is("windows") then
+		if not os.getenv("SDL2_DIR") then
+			print("Set SDL2_DIR enviroment variable.")
+		end
+	end
+end
+
 function exampleProject(_name)
 
 	project ("example-" .. _name)