Normally packagers should try to use ./configure --with-guestfs-path=...
However if they do not, then a regression in 1.44 caused the path to
constain the literal string "${exec_prefix}". This was because of
autoconf stupidity.
Try to resolve this by expanding ${exec_prefix} to something sensible.
The expansion will often be wrong (but at least it will now be a valid
path), hence the recommendation to always set --libdir and/or
--with-guestfs-path when configuring.
Fixes: commit 94ff24b880
Fixes: https://bugzilla.redhat.com/1915646
fb7b683a29