build: Change default for -Dman-pages from disabled to auto

Previously, `-Dman=false` was the default, because the generated man
pages were shipped in the distribution tarball already, so the option
actually mostly controlled whether to *re*build them.

The generated pages are no longer shipped in the tarball (and probably
haven’t been since the port to Meson, though I haven’t checked), so it
makes sense to change the default to encourage building the man pages if
the right tooling (`rst2man`) is available.

Signed-off-by: Philip Withnall <pwithnall@gnome.org>

.gitlab-ci.yml

@@ -11,7 +11,7 @@ cache:
     - _ccache/
 
 variables:
-  FEDORA_IMAGE: "registry.gitlab.gnome.org/gnome/glib/fedora:v25"
+  FEDORA_IMAGE: "registry.gitlab.gnome.org/gnome/glib/fedora:v26"
   COVERITY_IMAGE: "registry.gitlab.gnome.org/gnome/glib/coverity:v7"
   DEBIAN_IMAGE: "registry.gitlab.gnome.org/gnome/glib/debian-stable:v19"
   ALPINE_IMAGE: "registry.gitlab.gnome.org/gnome/glib/alpine:v3"