libxml2: Post process xml2-config file to actually work and bump version

Version bumped to 2.6.31
(2.6.32 has .rej files so patch target fails - upstream need to be informed).

The xml2-config file in staging dir needs to have prefix / exec-prefix setup
to understand the staging directory location.

This then means that packages that call xml2-config to get include paths/cflags
and libs etc work properly. 

This is very similar to what freetype does to freetype-config.