contrib: Ensure cmake won't interpret the provided prefix

We already provide an absolute path, which cmake might try to interpret
and convert to an equivalent value.
This works, except when we try to replace the prefix we provided by
@@CONTRIB_PREFIX@@ in change_prefix.sh, which leads to .pc files being
present but unusable

(cherry picked from commit 512dd949)
Signed-off-by: Hugo Beauzée-Luyssen <hugo@beauzee.fr>