Fix vapi search order
Fix vapi search order

- user-specified directories
- compiled-in path
- system data dir

With the old behaviour, a user-installed valac first picked up the, possibly
incompatible, vapis of a system-wide installation.

Signed-off-by: Michael Wild <bc730dcdf4596f222635a3dc0bdeff6087aaf636@users.sourceforge.net>

themiwi/vala,themiwi/vala,themiwi/vala