Make the GTK viewer use libxml2
Update git.mk
Stop automake from complaining about portability

I only care about GNU make anyway.
A cleaner way to split version number
Don’t build GMetaDOM frontend by default

The GTK viewer no longer depends on it, and GMetaDOM headers are causing
a lot of warning with my GCC version.
Finish porting the GTK viewer to libxml2
MathML 3 allows for href attributes

No need for the xlink namespace anymore.