Permit building python bindings separately from gz-math library (backport #636)

[mergify]

🎉 New feature

Part of osrf/homebrew-simulation#2834

Summary

This allows the src/python_pybind11/CMakeLists.txt file to be built as a top-level cmake project against an external gz-math library. The first commit (646de60) is a patch from @traversaro used in conda, and the second commit (d2bae80) replaces the GZ_LIB_INSTALL_DIR cmake variable with CMAKE_INSTALL_LIBDIR (see GzPackaging.cmake:111) since GZ_LIB_INSTALL_DIR is not defined with the minimal cmake project added in 646de60.

Test it

1. Invoke cmake on the root CMakeLists.txt with using -DSKIP_PYBIND11=ON to build and install the gz-math library without python bindings.
2. Invoke cmake on src/python_pybind11/CMakeLists.txt with -DPython_EXECUTABLE=/path/to/python to build and install python bindings for a given python version
3. Repeat 2 with for each desired version of python

I have a draft of an updated homebrew formula using this branch in osrf/homebrew-simulation#2836

Checklist

• Signed all commits for DCO
• Added tests
• Added example and/or tutorial
• 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

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

---

This is an automatic backport of pull request #636 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 17deea2 has failed:

On branch mergify/bp/gz-math7/pr-636
Your branch is up to date with 'origin/gz-math7'.

You are currently cherry-picking commit 17deea2b.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   src/CMakeLists.txt
	modified:   tutorials/installation.md

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   CMakeLists.txt
	both modified:   src/python_pybind11/CMakeLists.txt

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[scpeters]

Cherry-pick of 17deea2 has failed:

On branch mergify/bp/gz-math7/pr-636
Your branch is up to date with 'origin/gz-math7'.

You are currently cherry-picking commit 17deea2b.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   src/CMakeLists.txt
	modified:   tutorials/installation.md

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   CMakeLists.txt
	both modified:   src/python_pybind11/CMakeLists.txt

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

I've resolved the conflicts in a48b893

[scpeters]

to unbreak CI while this awaits review, I've opened osrf/homebrew-simulation#2840 to apply these patches to the gz-math7 formula

[azeey]

The top level CMakeLists.txt file has a minimum version of 3.10.2. Is this necessary? If we require 3.22.1 to be able to run this, we should document the requirement in installation.md

[scpeters]

the CMAKE_REQUIRE_FIND_PACKAGE_pybind11 variable on line 10 requires cmake 3.22, so I used the same version as Ionic to simplify merges

I will update the documentation to indicate the required cmake version

[scpeters]

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