chore: make libxml optional

[Antonov548]

Closes #1217

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[krlmlr]

Thanks! Can you please update the SystemRequirements section in DESCRIPTION too, also for GLPK?

[szhorvat]

If libxml2 detection fails at either of the three steps, could we print a message saying something like "GraphML import will not be available." ?

[szhorvat]

Also, it would be good if someone could test this on a system where libxml2 is not available: make sure it compiles, links, and trying to import a GraphML file gives a nice error. I don't see any reason why it wouldn't work, but just in case ... it's hard for me to hide libxml2 since it comes as standard on macOS.

[Antonov548]

Also, it would be good if someone could test this on a system where libxml2 is not available: make sure it compiles, links, and trying to import a GraphML file gives a nice error. I don't see any reason why it wouldn't work, but just in case ... it's hard for me to hide libxml2 since it comes as standard on macOS.

I have tried to delete libxml2 on my local virtual env. It compiles and links.

Didn't try to import file, but I will do.