Adds the ability to pass a WINDOWING_LIB variable to 'make' which set…

…s `--with-sdl` or `--with-glfw` (bkaradzic#2138)

* Adds the ability to pass a WINDOWING_LIB variable to 'make' which sets --with-sdl or --with-glfw

* Adds the ability to pass extra GENie args to 'make'

makefile

@@ -24,7 +24,7 @@ endif
 # $(info $(OS))
 
 BX_DIR?=../bx
-GENIE?=$(BX_DIR)/tools/bin/$(OS)/genie
+GENIE?=$(BX_DIR)/tools/bin/$(OS)/genie $(EXTRA_GENIE_ARGS)
 NINJA?=$(BX_DIR)/tools/bin/$(OS)/ninja
 
 .PHONY: help