Make xsltproc and xmlint optional when building from git

[robe]

Right now people can build from our tar balls cause comments are pre-built but much of our testing etc expects to have comments available or buildable.

This requires at least xsltproc.

While comments are great to make sure people properly documented new features, it does cause casual developers some angst. so I think this should be optional and comments install should be skipped when not present.

[robe]

I'm going to test this out. Might be already done a while back ago and the only issue left maybe was the extensions building requiring comments.

[Sandro Santilli <strk@…>]

In 10ca822/git:

Don't try building comments if they cannot be built

Closes #5543