No need to edit config.mk to set addictional CFLAGS/LDFLAGS or change the DESTDIR etc. now

This fixes imlib2 warnings (and occasional X errors/segfaults), which used to
occur when all files in feh's filelist were suddenly deleted by an external
application. Now, feh simply quits with "No more slides in show" (which makes
more sense, IMHO).

Just in case, and we don't want the compiler to complain about it, do we? ;)

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 :-)