message: fix sdl2-config argument
  The current version of SDL2 (2.0.9) doesn't have a `--static-libs` argument.
  Let's just link normally instead, and accept the runtime dependency on SDL2.
diff: @@ -29,7 +29,7 @@ VERBOSE ?= 0
  COMPILER = $(shell $(CC) -v 2>&1 | grep -q "clang version" && echo clang || echo gcc)
  SDL2FLAGS=$(shell sdl2-config --cflags)
  -SDL2LIBS=$(shell sdl2-config --static-libs)
  +SDL2LIBS=$(shell sdl2-config --libs)
  ifeq ($(USE_GTK3), 1)
  GTK3FLAGS=$(shell pkg-config --cflags gtk+-3.0) -DUSE_GTK3
  