Use pkg-config to locate fribidi library's files

[mhdna]

When using something like NixOS or the Nix package manager that place packages' files in their own custom paths, directly specifying the path to the fribidi library makes the compilation fail. Using pkg-config fixes that.

[bakkeby]

pkg-config is used for the pango patch as well so I don't see any reason to object to this. Will merge, thanks.