build/app: honor LFLAGS
build: staticly link libSDL2, work around weird link problem on ubuntu 12.04.3

- sdl2-config --static-libs doesn't actually get me linked against static libs
- just hardcode the real static path for now
- for some reason even though I have NVIDIA GL installed/selected, this box
keeps trying to link against mesa GL which lacks newer APIs.  a -L/usr/lib before
-lGL "fixes" it for now.