qdoc: relative paths in 'includepaths' variable are resolved too late

A potential issue with how QDoc handles include paths; typically, we have extra include paths in the .qdocconf:

includepaths += ./include

Where the . refers to the directory where the .qdocconf is located. This works because QDoc sets the current working directory to the directory of the file it's parsing.

However, an attempt to resolve those paths is made only after the entire config has been processed - at this point, the cwd may have changed. One such case is a 'wrapper' qdocconf (in another dir) that include()s the original to override some settings.