Allow cmake tarball archive to be disabled

[mattparks]

No description provided.

[icculus]

Is this because you want to use the CMake project in a subdirectory of a larger project, and having a target named "dist" is causing problems?

Honestly, since releases are just tagged in git and GitHub does the actual tarballs now, we should probably just remove it.

[madebr]

SDL conditionally disables installation, and distribution-related options, when it is built as a subdirectory.
The detection happens here.

Also, I would drop the if(UNIX). If it's up to me, Windows users can also create a distribution. But icculus isn't :)

[icculus]

@madebr When you have time and are bored, let's make this work like SDL does it. It's not urgent, though.

[madebr]

@madebr When you have time and are bored, let's make this work like SDL does it. It's not urgent, though.

I disabled source tarball creation from CMake completely in SDL.
Right now, you can create a source tarball through the build-scripts/build-release.py script:

build-scripts/build-release.py --actions source

[icculus]

Okay, with the dist target gone, we're good to go here without this PR. Thanks!