m4/virt-gnutls: use AC_CHECK_FUNCS to detect availability of functions

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>

m4/virt-gnutls.m4

@@ -60,8 +60,8 @@ AC_DEFUN([LIBVIRT_CHECK_GNUTLS],[
 
     OLD_LIBS="$LIBS"
     LIBS="$LIBS $GNUTLS_LIBS"
-    AC_CHECK_FUNC([gnutls_rnd])
-    AC_CHECK_FUNC([gnutls_cipher_encrypt])
+    AC_CHECK_FUNCS([gnutls_rnd])
+    AC_CHECK_FUNCS([gnutls_cipher_encrypt])
     LIBS=$OLD_LIBS
   fi
 ])