devel/pkgconf: accept share/pkgconfig

Since pkg-config then pkgconf was introduced, we as a project decided
the lib/pkgconfig was not suitable and moved all .pc files and installed
them into libdata/pkconfig instead.
This concerns the .pc file referring to potentially architecture dependant
information.
One thing that has been missed was that there may be some arch independant
.pc files only (like headers only libraries for example etc) then in
pkg-config but also in pkgconf it has been advertise to use share/pkgconfig
for those specific files. As this matches what hier(7) says about "share",
let's accept that in the default search path as well.

Reported by: Andrea Pappacoda <andrea@pappacoda.it>