bootstrap: Use an absolute path for ACBOOTDIR.

bootstrap: Use an absolute path for ACBOOTDIR.

If we use a relative path for ACBOOTDIR, Automake can’t tell the
difference between Autoconf’s configure script’s aclocal.m4
inclusions (…/m4/*.m4) and the guts of Autoconf
itself (…/lib/autoconf/*.m4) so it puts both of them into
$(am__aclocal_m4_deps).  This would be harmless, except that the
guts-of-Autoconf files are named *relative to $ACBOOTDIR*, which
means Make can’t find them later.  And this is why a build from a
clean git checkout always starts by regenerating aclocal.m4 and
configure again.

Using an absolute path for ACBOOTDIR gives automake enough of a clue
what’s going on (see the heuristic circa 5500 of current automake.in,
commented “Some modified versions of autoconf don’t use frozen files…”)
for it to produce the same value for $(am__aclocal_m4) that it would
if we were running an installed Autoconf.