maint: avoid bootstrap warning

maint: avoid bootstrap warning

I noticed that in a fresh checkout, autogen.sh generated the
following output, but continued on with execution:

autoreconf: running: automake --add-missing --copy --force-missing
gnulib/tests/Makefile.am:28: TESTS was already defined in condition TRUE, which includes condition WITH_EXPENSIVE_TESTS ...
gnulib/tests/gnulib.mk:28: ... `TESTS' previously defined here
gnulib/tests/Makefile.am:19:   `gnulib/tests/gnulib.mk' included from here

and after the run, line 28 of gnulib.mk lists GNULIB_TESTS, not TESTS.
After more investigation, I found that it is because gnulib bootstrap
provides two hooks, one before automake, and the other after; we used
the one that ran after, and were then rerunning automake ourselves;
and the warning was from the first run.  But a manual second run is
pointless if we use the right hook in the first place.

The wrong function name has been latent since commit 38c9440, and we
tried to work around it in commit 6cbab7c, but it took commit 70363ea
to finally change output enough for me to realize the root cause.

* bootstrap.conf (bootstrap_epilogue): Rename...
(bootstrap_post_import_hook): ...so that it gets run before automake.

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