[ign ➡️ gz] Internal parsed variables

[chapulina]

• Part of CMake API from ign to gz gazebo-tooling/release-tools#719

Summary

This PR updates just the variables parsed by cmake_parse_arguments to use gz instead of ign. Those variables are internal to each function and shouldn't affect the public API.

⬅️ This PR can be backported to ign-cmake2 later to reduce the diff between the branches.

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

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.

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[methylDragon]

A test build of packages up to ignition-transport12 failed with this version of CMake3, throwing...

This error

:(

I'm not sure what's up, but it might be something wonky with macro substitutions, or worse.., an erroneous macro-set variable that bubbles up into the parent scope that is now causing issues because of the gz change??

[methylDragon]

https://github.com/ignitionrobotics/ign-cmake/pull/245/files#diff-955d1f97e2360702839ba9277cc71b9797e65d417d5cb88f383fec0e53ad5124R1000
Found it! Line 1000:
_ign_handle_cxx_standard(ign_create_core_library -> _ign_handle_cxx_standard(gz_create_core_library

(:

_ign_handle_cxx_standard is a macro that looks for a prefix, not a macro/function name, so we just need to pass in that prefix so it can resolve the appropriate arg!

It builds! (The stderr from math is just the overlay/override check, no worries on that.)

[chapulina]

Found it! Line 1000:

Thanks! Well spotted! Committed in e897659

[methylDragon]

👍 pending green CI