librsvg: add 2.40.21, which does not require rust (#34585)

* librsvg: add 2.40.21, which does not require rust and has some security backports

https://download.gnome.org/sources/librsvg/2.40/librsvg-2.40.21.news

* librsvg: prevent finding broken gtkdoc binaries when ~doc is selected.

On my CentOS7 hosts, ./configure finds e.g. /bin/gtkdoc-rebase even when
~doc is selected.  These tools use Python2, and fail with an error:
"ImportError: No module named site"

So prevent ./configure from finding these broken tools when not building
the +doc variant.

var/spack/repos/builtin/packages/librsvg/package.py

@@ -16,12 +16,13 @@ class Librsvg(AutotoolsPackage):
     version("2.50.2", sha256="6211f271ce4cd44a7318190d36712e9cea384a933d3e3570004edeb210a056d3")
     version("2.50.0", sha256="b3fadba240f09b9c9898ab20cb7311467243e607cf8f928b7c5f842474ee3df4")
     version("2.44.14", sha256="6a85a7868639cdd4aa064245cc8e9d864dad8b8e9a4a8031bb09a4796bc4e303")
+    version("2.40.21", sha256="f7628905f1cada84e87e2b14883ed57d8094dca3281d5bcb24ece4279e9a92ba")
 
     variant("doc", default=False, description="Build documentation with gtk-doc")
 
     depends_on("gobject-introspection", type="build")
     depends_on("pkgconfig", type="build")
-    depends_on("rust", type="build")
+    depends_on("rust", type="build", when="@2.41:")
     depends_on("gtk-doc", type="build", when="+doc")
     depends_on("cairo+gobject")
     depends_on("gdk-pixbuf")
@@ -52,4 +53,16 @@ def setup_run_environment(self, env):
         env.prepend_path("XDG_DATA_DIRS", self.prefix.share)
 
     def configure_args(self):
-        return ["--enable-gtk-doc=" + ("yes" if self.spec.variants["doc"].value else "no")]
+        args = []
+        if "+doc" in self.spec:
+            args.append("--enable-gtk-doc")
+        else:
+            args.extend(
+                [
+                    "--disable-gtk-doc",
+                    "GTKDOC_MKPDF=/bin/true",
+                    "GTKDOC_REBASE=/bin/true",
+                    "GTKDOC_CHECK_PATH=/bin/true",
+                ]
+            )
+        return args