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: NPavel Hrdina <phrdina@redhat.com>

Signed-off-by: NPavel Hrdina <phrdina@redhat.com>

The LIBVIRT_RESULT does a nice formatting of the output and ensures
that the format is unified.
Signed-off-by: NPavel Hrdina <phrdina@redhat.com>

Unconditionally use gnulib's getopt module. This is needed by the bhyve driver
to provide a reentrant interface for getopt.

Several gnulib headers rely on features.h being included by ctype.h to provide
__GNUC_PREREQ, but on systems without glibc, this is not provided. In these
cases __GNUC_PREREQ gets redefined to 0, which causes build errors from checks
in src/internal.h.
Therefore, define __GNUC_PREREQ as early as possible. config-post.h is probably
the first header that is included, before any other headers.

bhyveload and bhyvectl wouldn't be checked otherwise as the configure
script wouldn't execute one of the tests:

checking for bhyve... /usr/local/sbin/bhyve
checking for bhyvectl... /usr/local/sbin/bhyvectl
checking for bhyveload... /usr/local/sbin/bhyveload
./configure: line 62602: test: too many arguments

Fix the shell statement testing the 3 binaries.

At this point it has a limited functionality and is highly
experimental. Supported domain operations are:

  * define
  * start
  * destroy
  * dumpxml
  * dominfo

It's only possible to have only one disk device and only one
network, which should be of type bridge.