Also manage `pkg-config` `.pc` files

[osirisinferi]

For some software I needed libshumate to generate the vala API, but it didn't find vapigen, even though eselect-vala did manage the /usr/bin/vapigen symlink.

Turns out when I symlinked /usr/lib64/pkgconfig/vapigen-0.56.pc to /usr/lib64/pkgconfig/vapigen.pc, it worked flawlessly.

I don't have any idea how eselect works internally, but would it be possible to also manage the /usr/lib64/pkgconfig/vapigen.pc symlink perhaps?