[CMake] Set rpath for individual targets and not globally

[guitargeek]

When using the rpath=ON option, the global CMake variable CMAKE_INSTALL_RPATH is mutated, which is problematic in several corner cases.

Furthermore, the path is set to match the install tree, resulting in nonsensical relative rpath for the build tree if the directory structure is different (e.g. because of gnuinstall=ON).

This commit suggests to append the rpaths at the target property level, and also set the rpaths for the build tree separately so they make sense.

New ROOT macros are introduced for this rpath setting, which are also used for the PyROOT libraries, which also need custom rpaths.

Closes #19327 and #19134.

Here are some rpaths for relevant libraries and executables with gnuinstall=ON and rpath=ON, before and after this PR:

root_install/bin/root
 - before: $ORIGIN:$ORIGIN/../lib/root
 - after : $ORIGIN/../lib/root

root_build/bin/root
 - before: :::::::::::::::::::::::::::
 - after : :::::::::::::::::::

root_install/lib/root/libThread.so
 - before: $ORIGIN:$ORIGIN/../lib/root
 - after : $ORIGIN/

root_build/lib/libThread.so
 - before: /home/rembserj/code/root/root_build/lib:
 - after : /home/rembserj/code/root/root_build/lib:

root_install/site-packages/libcppyy.so
 - before: $ORIGIN/../lib/root
 - after : $ORIGIN/../lib/root

root_build/lib/libcppyy.so
 - before: $ORIGIN/:/home/rembserj/code/root/root_build/lib:
 - after : /home/rembserj/code/root/root_build/lib:

root_install/site-packages/ROOT/libROOTPythonizations.so
 - before: $ORIGIN/../../lib/root
 - after : $ORIGIN/../../lib/root

root_build/lib/ROOT/libROOTPythonizations.so
 - before: $ORIGIN/../:/home/rembserj/code/root/root_build/lib:
 - after : /home/rembserj/code/root/root_build/lib:

root_build/math/mathcore/test/libTrackMathCoreUnitDict.so
 - before: /home/rembserj/code/root/root_build/lib
 - after : /home/rembserj/code/root/root_build/lib

[github-actions]

Test Results

    19 files      19 suites   3d 4h 23m 44s ⏱️
 3 220 tests  3 220 ✅ 0 💤 0 ❌
59 579 runs  59 579 ✅ 0 💤 0 ❌

Results for commit 7af2b1d.

♻️ This comment has been updated with latest results.

[hageboeck]

LGTM, but I added a few more suggestions.

[hahnjo]

I think this PR results in a breaking change that root-config --libs no longer adds -Wl,-rpath, which was quite convenient for building quick test programs...