Do not redefine GZ_PYTHON_INSTALL_PATH if it is already defined

[traversaro]

🎉 New feature

Summary

When doing packaging, it is sometimes useful for advanced users to just tell the build system where to install Python files, without any sort of auto-detection. This was already supported elsewhere (see

gz-msgs/core/CMakeLists.txt

Line 69 in 0472ba0

if (NOT GZ_PYTHON_INSTALL_PATH)

and https://github.com/gazebosim/gz-msgs/blob/gz-msgs10/cmake/gz_msgs_generate.cmake#L273) but was not supported at the root level.

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.

[codecov]

Codecov Report

Merging #393 (019129f) into gz-msgs10 (0472ba0) will not change coverage.
The diff coverage is n/a.

❗ Current head 019129f differs from pull request most recent head e2cc2ab. Consider uploading reports for the commit e2cc2ab to get more accurate results

@@            Coverage Diff             @@
##           gz-msgs10     #393   +/-   ##
==========================================
  Coverage      97.26%   97.26%           
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  Files             27       27           
  Lines           1170     1170           
==========================================
  Hits            1138     1138           
  Misses            32       32