Prefer clang over gcc on FreeBSD and OpenBSD

[vassilit]

Those platforms ships Clang in their base system.

GCC is somehow supported, but lag behind.

For the record, to test on FreeBSD, I had to:

• install py312-build and py312-setuptools,
• install package gmake and change all hard-coded occurences of 'make' to 'gmake' (SCons does not support $MAKE convention).
• install py312-sphinx-book-theme and py312-pillow (for rst2pdf, required even if the build log shows SKIP_PDF=1).

Contributor Checklist:

• I have created a new test or updated the unit tests to cover the new/changed functionality.
• I have updated CHANGES.txt and RELEASE.txt (and read the README.rst).
• I have updated the appropriate documentation

[vassilit]

Related to #4886

[vassilit]

I think (I have not seen them) that I overlooked side-effects of creating a new platform. There are others platform == 'posix' checks in the code.

[vassilit]

Not too much.

[vassilit]

One error is:

The license for Visual Studio has expired.
Your license for this product is expired and could not be refreshed. Please connect to the internet to continue to use this product.

The other one, I'm not sure. I've looked at TestSCons.run, TestCommon.run, TestCmd.run, etc
But I don't think the modifications I've made have anything to do with the stack size.
Are you sure all the testsuite is idempotent, including the GitHub CI environement ?

[mwichmann]

don't worry about these. the visual studio one is an artifact of the ci image at appveyor. since it frozen, they have to make a new one with the silly license refreshed. we heard they had, but not so in practice. the other is one of a small handful of intermittent fails. not your fault.