doc: mention how to set early defaults

doc: mention how to set early defaults

Jonathan Lebon reported an issue to me off-list about a regression
in libvirt's configure script, which I traced to a patch that
rearranged code that was checking $with_library compared to the
AC_ARG_WITH that actually set $with_library [1].  As the whole point
of the libvirt patch was to refactor code to make maintenance
easier by hiding the AC_ARG_WITH in a helper macro for a net
reduction in lines, it makes sense to actually document how to
check what value a variable has prior to the AC_ARG_WITH usage.

Alas, although the functionality for this has been present in
autoconf for ages, the documentation has been lacking.

[1] libvirt.org/git/?p=libvirt.git;a=commitdiff;h=654c709

* doc/autoconf.texi (Diversion support) <m4_divert_text>: Add
anchor.
(External Software) <AC_ARG_WITH>: Demonstrate how to use DEFAULTS
diversion, for earlier defaults.
(Package Options) <AC_ARG_ENABLE>: Likewise.
* THANKS: Add Jonathan Lebon.

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