contrib: change_prefix: handle relocatable prefix

Handle the special pkg-config variable ${pcfiledir}, this is meant to make .pc files relocatable. See:

https://gitlab.freedesktop.org/pkg-config/pkg-config/-/blob/d97db4fae4c1cd099b506970b285dc2afd818ea2/pkg-config.1#L400

Alternative solution to reverting 579a6ce3, which introduced ${pcfiledir} and currently breaks the CI on master.