Update README to mention pkg-config and current configure

Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>
Fix typo I introduced in the bdev change.

When adding the missing return value in Caglar's change (as discussed on
the mailing-list), I set err = -1 instead or ret = -1, causing an
obvious build failure...

Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>