Use relative install paths for plugin shared libraries

[azeey]

🦟 Bug fix

Summary

Fixes an error when building https://github.com/gazebo-release/gz_physics_vendor/ in the ROS buildfarm.

Similar to gazebosim/gz-tools#137

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.35%. Comparing base (2bc19ad) to head (ede7dd5).
Report is 1 commits behind head on gz-physics7.

❗ Current head ede7dd5 differs from pull request most recent head a141a08. Consider uploading reports for the commit a141a08 to get more accurate results

Additional details and impacted files

@@             Coverage Diff              @@
##           gz-physics7     #616   +/-   ##
============================================
  Coverage        78.35%   78.35%           
============================================
  Files              140      140           
  Lines             8079     8079           
============================================
  Hits              6330     6330           
  Misses            1749     1749           

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[traversaro]

Note that GZ_PHYSICS_ENGINE_INSTALL_DIR is used in many places (see https://github.com/search?q=repo%3Agazebosim%2Fgz-physics%20GZ_PHYSICS_ENGINE_INSTALL_DIR%20&type=code), and I am not sure if all those uses work fine with GZ_PHYSICS_ENGINE_INSTALL_DIR being a non-absolute path (in particular see

gz-physics/include/gz/physics/config.hh.in

Line 31 in 4e400e0

#define GZ_PHYSICS_ENGINE_INSTALL_DIR _Pragma ("GCC warning \"'GZ_PHYSICS_ENGINE_INSTALL_DIR' macro is deprecated, use gz::physics::getEngineInstallDir() function instead. \"") "${GZ_PHYSICS_ENGINE_INSTALL_DIR}"

, that even if deprecated is a public macro). Perhaps it could make sense to change only the places where DESTINATION argument of install to use GZ_PHYSICS_ENGINE_RELATIVE_INSTALL_DIR instead of GZ_PHYSICS_ENGINE_INSTALL_DIR ?

[azeey]

Note that GZ_PHYSICS_ENGINE_INSTALL_DIR is used in many places (see https://github.com/search?q=repo%3Agazebosim%2Fgz-physics%20GZ_PHYSICS_ENGINE_INSTALL_DIR%20&type=code), and I am not sure if all those uses work fine with GZ_PHYSICS_ENGINE_INSTALL_DIR being a non-absolute path (in particular see

gz-physics/include/gz/physics/config.hh.in

Line 31 in 4e400e0

#define GZ_PHYSICS_ENGINE_INSTALL_DIR _Pragma ("GCC warning \"'GZ_PHYSICS_ENGINE_INSTALL_DIR' macro is deprecated, use gz::physics::getEngineInstallDir() function instead. \"") "${GZ_PHYSICS_ENGINE_INSTALL_DIR}"

, that even if deprecated is a public macro). Perhaps it could make sense to change only the places where DESTINATION argument of install to use GZ_PHYSICS_ENGINE_RELATIVE_INSTALL_DIR instead of GZ_PHYSICS_ENGINE_INSTALL_DIR ?

Good catch. I've used GZ_PHYSICS_ENGINE_RELATIVE_INSTALL_DIR in a141a08.