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
cmake-3.0.1.tar.gz (5.3M)
=> cmake-3.0.1 (46M)

<pre>
$ time ./configure --prefix=$HOME/cmake
[...]
real	1m4.915s
user	0m51.245s
sys	0m8.609s
</pre>

<pre>
$ time make
[...]
real	1m24.187s
user	1m13.318s
sys	0m8.287s
</pre>

=> build tree: 203M
=> install tree: 40M