QDoc: Implement -installdir CLI option for module cross-linking.

QDoc: Implement -installdir CLI option for module cross-linking.

QDoc needs to know the final location of the installed documentation to
generate correct relative links between the modules. Normally you can
use QLibraryInfo::DocumentationPath for this, but since QDoc gets compiled
during Qt bootstrapping QLibraryInfo is not available yet.
The -installdir option still allows us to specify
QLibraryInfo::DocumentationPath  on the command line.

Change-Id: Ic4729f4daad112f0d175931467cf09cfcf5145a3
Reviewed-by: Martin Smith <martin.smith@nokia.com>