CMake cleanup and fixes

[SteelT1]

• Replace calls to deprecated exec_program with execute_process when using uninstall target
• Add comment for uninstall target
• Only install slade.pk3 if BUILD_PK3 is enabled (fixes The sudo make install / sudo ninja install fails because of slade.pk3 file missing #1641)

[Pedro-Beirao]

To add to this, any reason BUILD_PK3 is not ON by default?

[sirjuddington]

I turned it off by default as I was getting sick of it rebuilding the entire pk3 every time I built SLADE during development (since I'm using cmake in windows now instead of the VS project/solution). It could probably be turned on by default if there was a way to have it only rebuild the pk3 if any files changed in dist/res but I'm not sure if that's possible

[Wohlstand]

You may want to set the tree of files as dependencies, and have the pk3 build to be a target with pk3 file as a byproduct. I have in my projects a similar thing but with different purposes, but sense is same: source a pile of files and final file as a result of process.