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>