Enable easy swithing between Box2D and chipmunk in linux and nacl

The ENABLE_BOX2D/ENABLE_CHIPMUNK defines were previously
hardcoded in the Makefiles.  Now you get chipmunk by default
on both platforms and can enble Box2D by setting USE_BOX2D in
your environment.

Also remove erroneous -D__CC_PLATFORM_FILEUTILS_CPP__ and
-D__CC_PLATFORM_IMAGE_CPP__ from linux Makefiles.  These should
never be defined globally like this.