bootstrap: warn about BSD toolchain snafu

bootstrap: warn about BSD toolchain snafu

With my help on IRC, Roman Bogorodskiy identified a bug where the
BSD autoconf wrapper script eats stdin[1], which in turn causes
autopoint from gettext 0.18.3 to fail to find AM_GNU_GETTEXT_VERSION
out of configure.ac[2], with this error:

./bootstrap: autopoint --force
autopoint: *** Missing version: please specify in configure.ac through a line 'AM_GNU_GETTEXT_VERSION(x.yy.zz)' the gettext version the package is using

[1] http://www.freebsd.org/cgi/query-pr.cgi?pr=ports/184479
[2] http://git.sv.gnu.org/cgit/gettext.git/commit/gettext-tools/misc/autopoint.in?id=eea2dddde

The workaround would be painful to automate (because gettext
hardcodes the use of autom4te rather than honoring ${AUTOM4TE},
we'd have to create a temporary wrapper earlier in $PATH that
forwards to the real autom4te-x.yz - basically, repeating the
logic of what a wrapper script was supposed to do).  Further,
while the broken autom4te definitely hurts gettext 0.18.3, it
tends to have no issues with gettext 0.18.2; and since the
problem has been identified, it will not be long before fixed
autom4te and/or improved gettext that is not so stubborn
about using exactly 'autom4te' will be available.  Therefore,
for now we just warn about the situation.

* build-aux/bootstrap: Alert users to a broken autom4te.

Signed-off-by: Eric Blake <eblake@redhat.com>