This is a bit awkward. A more elegant solution would have
ignored *all* headers and then *un-ignored* some of them
if some conditions were met.
Sadly, we cannot really ignore all headers and then "unignore"
them: that's not how arrays in Meson work.
https://bugzilla.gnome.org/show_bug.cgi?id=794557
By default, only build man pages and gtk-doc if the build-deps were
found. To force-enable, pass -Dwith-docs=yes and -Dwith-man=yes.
Also use a foreach loop for man pages instead of listing them all
manually
