glib-compile-schemas considers XDG_DATA_HOME in addition to
XDG_DATA_DIRS since GLib 2.53.2, but this is not mentioned in its manual
and might bring confusion to readers.
So they are consistent with the way we’re building man pages in other
projects, and because some people are allergic to XML.
This changes the build-time dependencies from `xsltproc` to `rst2man`,
and also takes the opportunity to change the `-Dman` Meson option from a
boolean to a feature (so you should use `-Dman-pages={enabled,disabled}`
now, rather than `-Dman={true,false}`).
Signed-off-by: Philip Withnall <pwithnall@gnome.org>
Helps: #3037