QDoc: Make included directory paths resolved as absolute during config parsing

QDoc needs to resolve the directory variables in included qdocconf files as absolute before they are added to the list of directories.

Example:
a.qdocconf
sourcedirs += ..somerelativepath $somevariable

b.qdocconf
include (a.qdocconf)
sourcdires += somepath

The above doesn't work at the moment.