autopoint: Allow user to specify custom autom4te executable
* autopoint.in (func_trace): Allow user to specify autom4te
executable name through $AUTOM4TE envvar.
* gettextize.in (func_trace): Likewise.
Suggested by Eric Blake at
<https://lists.gnu.org/archive/html/bug-gettext/2013-12/msg00006.html>.
