We rely on a specific minimum version of gtk-doc to be able to build the
GTK API reference for the new API. In order to be able to use gtk-doc as
a subproject, though, we need to use a recent version of Meson.
The 'documentation' option also guarded the man page build. Instead
if skipping the whole docs subdir skip the specific gtkdoc calls, so that the
man page build still works.
This brings it in line with the gtk3 meson build.
