Relocation works by recognizing paths in the loaders cache
which start with the built time prefix and extract the relative
path from that.

This leads to the following problem when updating the cache:

In case the package is build on another machine one has to
either match the build directory layout or adjust the
cache by hand for the resulting cache to stay relocatable.

This commonly occurs with msys2 where mostly pre-build packages
are used which are built on another machine and the cache gets
generated at install time. Another case is updating the cache
in a separate deployment environment.

This patch takes the package installation directory as a base
and writes relative paths into the cache when relocation
is enabled. When loading the cache a relative path is made
absolute by prepending the package base again.

https://bugzilla.gnome.org/show_bug.cgi?id=776081