autopoint: Select macro tracing method by Autoconf version
Reported and analyzed by Pádraig Brady in:
<http://lists.gnu.org/archive/html/bug-gnulib/2014-07/msg00071.html>.
* autopoint.in (func_trace_autoconf): Rename from func_trace.
($func_trace): Set to func_trace_autoconf if Autoconf >= 2.69 is
availabl, otherwise to func_trace_sed.
* gettextize.in: Likewise.
