Mk/Uses: Add new pathfix expression

While working on Apertium ecosystem it looks like their entire 500+
applications uses hardcoded pkgconfigdir with (prefix)/share/pkgconfig.
This patch adds that regex for easier porting for the huge pile of
ports.

Reviewed by: tcberner
Approved by: portmgr
Differential Revision: https://reviews.freebsd.org/D34780