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.