Correct CMake install locations on *nix

[perryprog]

Summary

None

Purpose of change

PR #61225 changed the location some stuff got installed in, which while correct for the Windows style install location, is incorrect for the default *nix style install locations. (CMAKE_INSTALL_DIR is by default /usr/local instead of c:/Program Files/${PROJECT_NAME}/ according to here.)

Describe the solution

Adjust these install locations to be correct for both OS's, hopefully. I'm not sure what the install locations for gfx/ and doc/ are intended to be for Windows, so I've set it to what it was previously, which is under DATA_PREFIX. I'd like it if this can be confirmed before the PR is merged so I can make the default install locations for both of those directories also depend on OS.

Describe alternatives you've considered

Testing

Builds and runs on macOS. I'd appreciate it if someone on Windows can test.

Additional context

CC @alef: I'd like to know what would be the correct directories according to what you were trying to do.

[alef]

The CMake builds do not install correctly, it should be fixed in #61880.
I suggest you rebase this PR after mine gets merged (@akrieger is back)

[akrieger]

I mean I'm still on vacation but I found time to hop on discord today :p

[perryprog]

Ah, thanks, I didn't realize you had opened the second PR already. Because install is effectively broken on *nix right now, do we want to merge this as a temporary fix? (I'm thinking it's find to wait considering I seem to be the first person to notice, unless there's an issue I missed.)

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[Night-Pryanik]

@perryprog could you please resolve conflicts?

[perryprog]

Oops, sorry—been busy with real life so I haven't had a chance to look at this PR. I'll double check that this is still an issue with the merge of #61880, and if so rebase off that.

[perryprog]

All done; sorry for the delay. @alef, can you confirm this still installs as expected for Windows? It looks good to me on macOS.