All configuration options (except for custom attributes which still dwell

All configuration options (except for custom attributes which still dwell
in lib/custom.h) are now stored in config.mk to make them available to both
makefiles (conditional linking etc.) and C programs (lib/autoconf.h is
generated from config.mk by a simple shell script).

This gives an easy way how to create special-purpose modules (like the
SQL gatherer) which need extra libraries -- just make them a compile-time
option ;)