build-sys: only install legacy *.pkla files with old polkit versions (#5555)

The legacy *.pkla files are not required when running against polkit ≥ 106,
and we want to avoid shipping files in /var if possible (but pkla files
can only be in /etc/ or /var).

Only install the *.pkla files if we detect an old polkit version during
configure. Don't install them if polkit isn't installed during build, as
distributions other than Debian-based ones have moved to the new polkit
long ago.

Fixes #5523