Add a --with-distro=ID argument for configure, so it is possible to
manually specify the distro to use for the packages (in case os-release
does not provide ID=.., or the ID is not recognized yet).
In the case when --with-distro is not set, keep doing the autodetection,
but using os-release only, i.e. dropping the checks for all the other
-release files -- since there is --with-distro, older distros with no
os-release can still be used.
RWMJ: Add documentation to guestfs-building(1).
531316cc3f