Make sure sharedir/extension/ directory is created when needed.

Make sure sharedir/extension/ directory is created when needed.

The previous coding worked as long as MODULEDIR wasn't set explicitly,
because we create sharedir/$(datamoduledir) and the default value of
that is "extension".  But if some other value is specified for MODULEDIR
then the installation directory needed for the control file wasn't made.

Cédric Villemain