Policy-Kit has been replaced by polkit (referred to, respectively,
as POLKIT0 and POLKIT1 in our Makefiles).
The last build fix with old Policy-Kit was in May 2013:
commit <442eb2ba> and build with -Wunused-label was broken
since April 2016: commit <8437130>
Signed-off-by: Ján Tomko <jtomko@redhat.com>
LIBVIRT_ARG_WITH_ALT is more generic than LIBVIRT_ARG_WITH, which
is tailored at switching features on and off.
Rename the macros according to their intended purpose, and add
some documentation to help developers pick between the two.
