The idea here is that patches that add items to these lists will often
be easier to rebase over other additions to the same lists, because
they won't be trying to touch the very same line of configure.in.

There will still be merge conflicts in the configure script, but that
can be fixed just by re-running autoconf (or by leaving configure out
of the submitted patch to begin with ...)

Implementation note: use of m4_normalize() is necessary to get rid of
the newlines, else incorrect shell syntax will be emitted.  But with
that hack, the generated configure script is identical to what it
was before.

Discussion: https://postgr.es/m/19344.1539050134@sss.pgh.pa.us