build: Unconditionally enable --enable-man configure option on autotools

We don’t ship the generated man pages in the ninja-generated tarball.
For consistency, we should do the same with autotools. This will
probably annoy a few people who didn’t want to install xsltproc in order
to locally build GLib, but all the distributions should be rebuilding
the documentation for packaging anyway, and they’re the people we care
about. People building GLib locally should be moving to Meson.

Signed-off-by: Philip Withnall <philip@tecnocode.co.uk>

GNOME/glib#1436
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