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
It doesn't matter if the prebuilt cmake is 32-bit, because it merely generates makefiles. A 64-bit system just needs 32-bit support libraries/runtimes for running the cmake executable, but the build product for a 64-bit compiler is, well, 64-bit. :) I'm just saying that if the 32-bit support is there but a non-admin user is missing cmake, grabbing the prebuilt binary must be the fastest and easiest way to get it.

We could fairly well write a helper script to assist with all this, and we can trigger it from configure if no cmake is detected. Something like `uname -m` and `uname -s` for detection and offer to download/extract/configure/build the appropriate package. I don't know if there's a nice and universal way to test the presence of 32-bit support libs on 64-bit systems, but we can offer to download a source package by default and let the user choose.