Unify cmake libxml build logic

[skef]

... and add environment variable to force static link of libxml2

Description

The GitHub windows build has been failing because CMake finds a stray, poorly
matching version of LibXML2. This PR centralizes the LibXML2 build logic and adds
two environment variables, one to force downloading and static linking against
the library one to force use of the system library. The former is used for GitHub
windows builds.

Checklist:

• I have followed the Contribution Guidelines
• I have added test code and data to prove that my code functions correctly
• I have verified that new and existing tests pass locally with my changes
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation

[kaydeearts]

👍 awesome!