Bugfix: CMake Install paths

[Davknapp]

Instead of hard-coding paths use GNUInstallDirs to automatically set installation dirs for bin, lib etc..

We hard coded some installation paths, while p4est and sc use GNUInstallDirs to automatically set these paths. This PR unifies the behaviour

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[KerstinHartung]

Thanks @Davknapp ! That fixes the issue of incorrect linking and two lib-directories I had encountered.

[spenke91]

Your solution may be completely right, but better safe than sorry:
According to CMake documentation, the RELATIVE_PATH argument of file works like this:

It seems to odd to me that we use two directories here instead; and wouldn't this mean relativePath would contain the relative path from the bin to the libfolder, so basically .../lib? 🤔

Or is this supposed to be some correction of the current state only?

[spenke91]

Should this be install( TARGETS ${ADD_T8_EXAMPLE_NAME} RUNTIME ) , like in the tutorials below?

[spenke91]

I tested it on the Teamsserver and it worked fine - however, it also did so before, so maybe that's not a great test. As you can see, I do have some question about the relative path 🤔

[spenke91]

Also, the PR description says it unifies the installation dirs with p4est and sc, but p4est does not seem to use GNUInstallDirs - do you think we might run into problems with that again at some point?