Use config.mk for configuration, both GNU make and FreeBSD make are supported.

This reverts commit 15ede265.

Apparently, the old code was based on libjpeg6. However, most systems use
libjpeg8 now, which has a few new features like cropping, leading to a lot
of fuckup with feh on libjpeg8 systems. Now feh works with libjpeg8,
but probably doesn't support libjpeg6 anymore.

Also, this code copypasta really sucks :-)