Colomban Wendling 78c6a2ad6d Fix check for GtkDoc header generation dependencies
The checks itself were correct, but the logic for the final decision
didn't properly handle `--enable-gtkdoc-header=auto`, which also
happens to be the default.

Also fix hiding error messages from Python when trying to import the
`lxml` module to see whether it's available or not.

Closes #929.
2016-03-03 14:49:37 +01:00
..