Improve appdata-xml.m4 deprecation notice

Use AU_DEFUN rather than AC_DEFUN to define the APPDATA_XML macro, so
that standard obsolete warning messages are emitted during an aclocal
run. Increment the serial to ensure that this appdata-xml.m4 takes
precedence over the one in appdata-tools.

https://github.com/hughsie/appstream-glib/issues/23
Merge pull request #24 from amigadave/master

Improve appdata-xml.m4 deprecation notice