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
8 jobs for 1436-rebuild-all-the-docs in 20 minutes and 11 seconds (queued for 4 seconds)
Status Job ID Name Coverage
  Build
passed #68593
cross-android_api21_arm64

00:01:36

passed #68594
cross-android_api28_arm64

00:01:36

passed #68595
cross-mingw64

00:02:11

passed #68592
fedora-x86_64

00:04:23

manual #68598
freebsd-11 allowed to fail manual
freebsd-11-x86_64
passed #68596
win32
msys2-mingw32

00:08:36

passed #68597
win32
vs2017-x64

00:06:04

 
  Coverage
passed #68599
coverage

00:01:46

73.4%