libproj: Add proper rpath for linux and macOS

[mwestphal]

This add proper rpath in order to be able to use libsqlite not from system.

• AI (Copilot or something similar) supported my development of this PR. See our policy about AI tool use. Use of AI tools must be indicated.
• Closes #xxxx
• Tests added
• Added clear title that can be used to generate release notes
• Fully documented, including updating docs/source/*.rst for new API

[mwtoews]

Have you checked if setting built-in -D CMAKE_BUILD_RPATH_USE_ORIGIN=ON? More docs on BUILD_RPATH_USE_ORIGIN.

I've tried it locally with Ubuntu, but I don't see anything with objdump -x /tmp/inst/lib/libproj.so | grep 'R.*PATH', so perhaps I'm missing something. I've also checked this PR too, also nothing found in libproj.so with objdump. This PR has expected:

$ objdump -x /tmp/pr4725/lib/libproj.so | grep 'R.*PATH'
  RUNPATH              $ORIGIN

[mwtoews]

I was unsuccessfully checking to see if built-in CMAKE variables could be used for this feature, but I can't see any. This PR does enable the feature. Could it be documented here?

[jjimenezshaw]

I've tried it locally with Ubuntu, but I don't see anything with objdump -x /tmp/inst/lib/libproj.so | grep 'R.*PATH',

Have you tried readelf -d /tmp/inst/lib/libproj.so ?

[mwtoews]

@jjimenezshaw yes, tried that too. I believe that readelf -d and objdump -x are equivalent for inspecting RPATH/RUNPATH.

[mwestphal]

BUILD_RPATH_USE_ORIGIN is for build rpath, not install rpath.

[mwestphal]

I was unsuccessfully checking to see if built-in CMAKE variables could be used for this feature, but I can't see any. This PR does enable the feature. Could it be documented here?

done.

[mwestphal]

I've been talking with CMake devs and this approach is wrong. -DCMAKE_INSTALL_RPATH should be enough to fix that locally and should not be handled by the project like that. Sorry for the noise.