Loader for BPG image format for Imlib2 (i.e. for Feh).

1. Ensure Imlib2 development things (like libimlib2-dev) are installed;
2. Build libbpg (for example, v0.9.3. Don't worry if you can't build bpgenc or bpgdec due to missing png or jpg libraries - we need only libbpg.a);
3. Adjust BPGDIR in makefile;
4. Build and install bpg.so.