[CMake] Append to CMAKE_INSTALL_RPATH without force-overriding cache.

[hageboeck]

@guitargeek, I would like to suggest this small change for #19135.
It's preferable not to touch any cache variables, as they are user-settable (and can be altered after configuring). Given that RootBuildOptions is included at the outermost scope, it's sufficient to declare a new variable with the same name as the cache.

On each CMake run, this variable will be recomputed from the cache variable + whatever we append, leaving the CMakeCache.txt invariant.

I also took the opportunity to clarify what the rpath option does, since this was not really explained (see e.g. #19327).

[guitargeek]

I also have an alternative solution that avoids messing around with the CMAKE_INSTALL_RPATH completely:

• [CMake] Set rpath for individual targets and not globally #19501

[github-actions]

Test Results

    20 files      20 suites   3d 3h 51m 26s ⏱️
 3 223 tests  3 223 ✅ 0 💤 0 ❌
62 798 runs  62 798 ✅ 0 💤 0 ❌

Results for commit 5ad21d5.

[hageboeck]

Closing in favour of #19501