gentoo: enable harfbuzz useflag

The following USE changes are necessary to proceed:
 (see "package.use" in the portage(5) man page for more details)
 required by x11-libs/pango-1.48.7::gentoo
 required by gnome-base/librsvg-2.50.7::gentoo
 required by gnome-base/librsvg (argument)
>=media-libs/freetype-2.10.4 harfbuzz

and fix circular dependencies

 * Error: circular dependencies:

(media-libs/harfbuzz-2.8.1:0/0.9.18::gentoo, ebuild scheduled for merge) depends on
 (media-libs/freetype-2.10.4:2/2::gentoo, ebuild scheduled for merge) (buildtime_slot_op)
  (media-libs/harfbuzz-2.8.1:0/0.9.18::gentoo, ebuild scheduled for merge) (buildtime)

 * Note that circular dependencies can often be avoided by temporarily
 * disabling USE flags that trigger optional dependencies.

gentoo

@@ -8,9 +8,10 @@ ENV ACCEPT_KEYWORDS="amd64" \
     FEATURES="ccache -sandbox -usersandbox -ipc-sandbox -mount-sandbox -network-sandbox -pid-sandbox" \
     CCACHE_SIZE="2G" \
     MAKEOPTS="-j9 -l8" \
-    USE="cairo introspection jpeg tiff"
+    USE="cairo harfbuzz introspection jpeg tiff"
 
 RUN emerge dev-util/ccache dev-lang/rust-bin \
+&&  USE="-harfbuzz" emerge media-libs/freetype media-libs/harfbuzz \
 &&  emerge app-text/poppler \
     dev-python/pycairo \
     dev-python/pygobject \