switch build system to CMake

[LeSpocky]

I propose switching the build system from Autotools to CMake. Here is why:

• I'm using CMake for years now and I'm familiar with the details how to use it
• I would have to learn Autotools from scratch to fix #5 or to make a new release tarball
• CMake supports cross compiling and is a modern build system supported by embedded build projects like ptxdist or buildroot
• I have already written basic CMake support in my personal fork: https://github.com/LeSpocky/libcgi/tree/cmake
• I have already written unit tests based on CTest which could be easily integrated then: https://github.com/LeSpocky/libcgi/tree/tests

Some things would still have to be done:

• support for pkg-config to make it easier for other projects to use libcgi
• use CPack to create a release tarball for milestone v1.1
• remove old Autotools files

[LeSpocky]

Speaking of pkg-config, I tried this but it doesn't seem to work properly yet. So the solution seems to create some template and install it with configure_file.

[LeSpocky]

Integrating pkg-config was easier than I thought. :-)