dev-libs/glib-2.58.1: actually fix build without dev-util/gtk-doc

dev-libs/glib-2.58.1: actually fix build without dev-util/gtk-doc

Our empty gtk-doc.make has to actually be near-empty and still declare
EXTRA_DIST and CLEANFILES, as that's what autogen.sh does to have it
working and it fails otherwise. Not sure how I managed to not notice
this during testing, as I'm pretty sure I tried without gtk-doc - maybe
a failure to delete workdir inbetween different tries.
Add note that we exceptionally do NOT need gtk-doc-am during build for
this package with USE=-gtk-doc

Bug: https://bugs.gentoo.org/673336
Closes: https://bugs.gentoo.org/673346
Signed-off-by: Mart Raudsepp <leio@gentoo.org>
Package-Manager: Portage-2.3.52, Repoman-2.3.11