Add a proper build system

This adds a proper build system, replacing the current 3 hackish build system that are platform dependent and mostly need to be edited in order to compile Namecoin, removing the triplication of the list of flags and files to compile.

It is based on autoconf and a Makefile-based build system I wrote (https://github.com/Midar/buildsys), offers the configure flags --without-miniupnpc and --without-qt, decides whether to build namecoind or Namecoin-Qt based on the latter flag and searches for all dependencies so that the user does not need to edit files anymore.

The build system is used by various projects already, including Audacious Media Player, almost all Atheme projects and almost all ObjFW-related projects, including ObjFW itself, so it is known to not only be easy to use for packagers, which was a design goal form the start, but also known to be highly portable, which even was the primary goal from the start, thus making it much more likely Namecoin will be imported into distributions or ported to new platforms.

I tested Namecoin with the new build system on OS X and Linux, I could not test it on Windows yet, as I currently don't have a Windows box, but I'm confident it will work. I'm also going to try it on all 3 major BSDs - this will even put Namecoin ahead of Bitcoin, which is still a major pain on for example NetBSD.

This PR also incorporates https://github.com/namecoin/namecoin/pull/108, as I had to use something that does compile as the base, of course.