Sigh, autoconf has AC_CHECK_FUNC and AC_CHECK_FUNCS and both check
whether some function exists or not.  However in addition to the obvious
difference that the later is able to check multiple functions it also
defines HAVE_FUNCTION_NAME for each function.

Commit 680d2f49 moved the code to separate file, but also blindly
changed the macro to AC_CHECK_FUNC without confronting the
documentation.
Reported-by: NJohn Ferlan <jferlan@redhat.com>

<https://www.redhat.com/archives/libvir-list/2016-December/msg01086.html>
Signed-off-by: NPavel Hrdina <phrdina@redhat.com>

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>

Following patch will update LIBVIRT_CHECK_(LIB|LIB_ALT|PKG) macros in
a way that you will be able to call a another macro as a fallback if
the first one fails.  To allow that, we need to move the
LIBVIRT_ARG_WITH out of those macro to not have two or more same lines
in output of "configure --help".
Signed-off-by: NPavel Hrdina <phrdina@redhat.com>

This fixes a build issue with old gnutls.
Broken by commit 680d2f49.
Reported-by: NOlga Krishtal <okrishtal@virtuozzo.com>
Signed-off-by: NPavel Hrdina <phrdina@redhat.com>

Commit d8a8af34 changed the minimal required version of gnutls
so it's safe to remove the code for older versions.
Signed-off-by: NPavel Hrdina <phrdina@redhat.com>

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