[LibXML2] Link LibXML2 Statically for Linux

[kaydeearts]

Description

Due to a bug in the linux python wheels involving dynamically linking libxml2, discovered in #1525 , we'll be installing libxml2 externally through ExternalLibXML2.cmake and linking statically, just like we do in most Windows cases.
We will revisit this in the future and attempt to fix dynamic linking for linux.

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

[skef]

Do we know this is a Linux problem as opposed to a wheel problem? If it's the latter we could just add an optional control (like checking an environment variable) to force static linking and then use that when building the wheels.

[sternenseemann]

Can this please be made configurable? When packaging afdko for nixpkgs we can provide libxml2 via the package manager and be confident that it'll be available at runtime as well. What we don't want is to download and link a random version of libxml2 in when building afdko.

[skef]

The Linux restriction in this release is likely temporary -- we needed to do something like this to resolve the libxml2 problems in the previous release. We will probably start to revisit the specifics week after next.

[skef]

@sternenseemann I added an environment variable to the root CMakeLists.txt that should allow you to force linking against the system LibXML2 on Linux. If you just set it to anything CMake should find it. Give it a shot on your distro and if it doesn't work file an issue against afdko and I'll look at it.

[sternenseemann]

@skef thanks that works & we are able to update to 3.9.2 now.