Adds a basic CMake based build system.

[OmniBlade]

No description provided.

[ghost]

The project already has support for four different compiler setups which should cover just about everyone. I don't think we really need to add anymore at this point. As a matter of fact it would be ideal to remove all but one, and go with CMake for the modern port.

Edit: and yeah I know it's annoying having four setups, but for the time being the project needs to remain compatible with VC6 while also giving those with modern tools a chance to build.

[OmniBlade]

I would suggest that while VC6 compatibility might be a sticking point for a single build generator system ideal, you could move closer to it by using CMake for all except the VC6 project which would be retained for backward compatibility and assembly verification builds. I'm quite happy to make further alterations/improvements to the CMake system if required such as adding tool chain files to make mingw builds easier and such.

[mewmew]

Thanks for the PR @OmniBlade. Using CMAKE definitey makes sense for modern C++ projects. The main focus of Devilution at this point is restoration (see #11). However, there are several forms actively developed, some of which use CMAKE. See for instance https://github.com/nomdenom/devilution/tree/miniwin-win, https://github.com/pengyz/devilution/tree/linux-port both of which use CMAKE.

There is also https://github.com/qndel/Infernity, https://github.com/diasurgical/devilutionX and https://github.com/diasurgical/devilution-plus-plus.

Closing this issue for now, as for diasurgical/devilution the current aim is for binary accuracy, something that all forks will benefit from. Later on when this is achieved, adding CMAKE would be great.