use frameworks for os x tiles by default

make frameworks default for os x, easier to have these installed than libsdl.
OSXLIBSDL=1 for libsdl, libsdl_ttf, omit for frameworks
OSX_MIN is OSXMIN (maybe should be VERMIN...)
add -I paths to handle both #include "SDL/SDL.h" and "SDL.h", etc.
SDLMain.o target inherits flags set in CXXFLAGS, but separately, also
picks up CFLAGS.