Switch to CMake

I've tested on Linux, OSX and Cygwin, but please help with testing your favorite configurations and report any issues found.

This change provides a configure wrapper script to make it almost backwards compatible with the old build system. See `configure --help` for details.

What comes to the build flags, it's not 100% identical with the old ones. Some of the platform specific build flags collected over the last decade were intentionally left behind. The latest ZNC requires a modern toolchain that supports C++11 anyway, so many of those old exotic flags are probably no longer even needed. The idea is to let CMake as a cross-platform to specify the correct flags, mostly.

The proposal also changes the structure a bit. After struggling with the damn object files for a good while, I decided to propose building the core as a library instead, like it was already done on Cygwin. I strongly believe this is the right direction to go in the future (see #210 and [Wiki](http://wiki.znc.in/Ideas)).

/cc @samuelgaist
> Probably it should have a comment with the used configure parameters.
> e.g. ../configure --enable-foo

Done: https://github.com/jpnurmi/znc/commit/00180b94d7ec2adb2d296893c10f082e4a63d49e

> Currently I get that info from config.log.
> Btw, when something fails, where to find why?

There's CMakeFiles/CMakeOutput.log and CMakeFiles/CMakeError.log, but they are not nearly as good as autotools' config.log...