This eliminates the build-time dependency on SDL_image and the run-time
dependencies on all libraries used by SDL_image except SDL and libpng.
In addition we now let libpng convert to ARGB format while decoding the
image, rather than letting SDL convert the surface afterwards.
It takes as its parameter the name of the targeted platform.
Currently, those are nanonote, dingux, pandora, gp2x and pc.
The makefiles will install the data accordingly to the platform specs
(e.g. install the 800x480 skins for the pandora platform).
