glib fix for Windows

[chapulina]

🦟 Bug fix

Summary

This is needed to compile ign-common3 on Windows. This patch is applied to the ignition-cmake2 package on conda-forge, as explained here. That's why our Windows source install tutorial tells users to build everything from source, except for ign-cmake. But we should be building everything from source.

I checked that this works on Windows, but I'll leave it as a draft until it can be verified on other platforms too.

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• 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

[chapulina]

@osrf-jenkins run tests again

[chapulina]

Opening for review, I think this shouldn't have adverse effects, because the worst that can happen is that the strings GLIB_INCLUDE_DIR-NOTFOUND and GLIBCONFIG_INCLUDE_DIR-NOTFOUND get appended to GTS_INCLUDE_DIRS. I think that adding invalid include paths shouldn't break anything.