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.
Usage of AC_REQUIRE will mess with order how LIBVIRT_CHECK_* macros
are composed into configure.ac. This ensures that the output of
configure --help is properly ordered and grouped into sections.
Signed-off-by: Pavel Hrdina <phrdina@redhat.com>
