Use relative cmake paths

[trexxet]

Replace CMAKE_SOURCE_DIR with relative path counterparts.

These changes should allow using the library as an external dependency with the FetchContent, as for that case CMAKE_SOURCE_DIR points to the top-level CMakeLists.txt.

[Bill-Gray]

Thank you! I know virtually nothing of cmake, and was hoping somebody more knowledgeable about it would step in here.

It appears not to have broken the AppVeyor builds (which use cmake; the Github Actions builds all rely on more 'traditional' Makefiles). Those builds don't test the actual ability to use the library with FetchContent. But I'd think that if this Works On Your Machine™ and with the AppVeyor builds, and no one points out problems, we could merge this and close the underlying issue.

[trexxet]

Thanks. There are a couple more things to fix (include paths etc.), so don't merge it now.

[trexxet]

Ok, seems to be working now, and CI is fine.

By the way, is there any reason to have PDC_SDL2_BUILD, PDC_SDL2_DEPS_BUILD & PDC_GL_BUILD enabled by default?

[GitMensch]

@juliusikkala @neko-para @jwinarske Can someone of you have a look at this PR?
(If two agree then I'd pull, but as I'm no cmake person...)

Note: I also think that the SDL + GL parts should be optional (needs an adjustment to appveyor.yml when conditional)

[juliusikkala]

Looks good to me, and I also agree that SDL + GL should be optional.

Although, it looks like I'm not able to build the project on my system anymore (OpenSUSE Tumbleweed). It's been a while since I last worked on my PDCursesMod-dependent project. I don't think these problems are caused by this PR; in fact, this removes some of the errors I see when trying to build master, so that's a good sign at least. It looks like the SDL & zlib deps need to be updated, as the old versions appear to not work with newer CMake versions that broke compatibility with CMake < 3.5. But that's not something that has to be dealt with in this PR, so I opened a new separate PR for updating those.

[GitMensch]

@Bill-Gray if you happen to be online, please cancel all older Appveyor builds as these take very long - and while I can start them with a push to a branch or by allowing the workflow to run, I can't do anything on Appveyor itself (if you want to, you should be able to adjust that at https://ci.appveyor.com/team)