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
Why those checks are done with C compiler instead of C++ compiler?

2014-09-18 21:45 GMT+01:00 J-P Nurmi <notifications@github.com>:

> In .travis.yml:
>
> >        global:
> >            - secure: "i2f2UVDnyHT/9z0U3XvgTj8eDERvnc1Wk7HpseEjb75JwGzqn/2R+RKHmoSrwK3hFgij2IMxZL19XtHFwMz9t5A/huAAKD74KMMI/QpeZEJ/sjT3CTLcE9HEVDdJOjc7dfLRxb2hZtgvx8clZIMrpeUdPhci8openff30KvXVbg="
> >  matrix:
> >      include:
> >          - compiler: clang
> > -          env: CFGFLAGS=--enable-debug MYCXXFLAGS="-fsanitize=address -O1 -fno-omit-frame-pointer -fno-optimize-sibling-calls -fPIE" MYLDFLAGS="-fsanitize=address -pie"
> > +          env: CFGFLAGS=--debug MYCXXFLAGS="-fsanitize=address -O1 -fno-omit-frame-pointer -fno-optimize-sibling-calls -fPIE" MYCFLAGS="-fsanitize=address -O1 -fno-omit-frame-pointer -fno-optimize-sibling-calls -fPIE" MYLDFLAGS="-fsanitize=address -pie"
>
> The checks whether getopt_long(), getphassphrase() exist are done with a
> C-compiler. Because of this, the C-language must be enabled for the
> project. This in turn means that CMake does a check for a working
> C-compiler. The compiler check picks up LDFLAGS and fails if there's no
> matching CFLAGS. There's no need to list these twice in this list, though.
> :p I'll clean it up.
>
> —
> Reply to this email directly or view it on GitHub
> <https://github.com/znc/znc/pull/648/files#r17753525>.
>