librsvg: add libgsf option

Closes Homebrew/legacy-homebrew#20855.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>

Formula/librsvg.rb

@@ -5,19 +5,24 @@ class Librsvg < Formula
   url 'http://ftp.gnome.org/pub/GNOME/sources/librsvg/2.36/librsvg-2.36.3.tar.xz'
   sha256 '3d7d583271030e21acacc60cb6b81ee305713c9da5e98429cbd609312aea3632'
 
+  depends_on :x11
   depends_on 'pkg-config' => :build
   depends_on 'xz' => :build
   depends_on 'gtk+'
   depends_on 'libcroco'
-  depends_on :x11
+  depends_on 'libgsf' => :optional
 
   def install
-    system "./configure", "--disable-dependency-tracking",
-                          "--prefix=#{prefix}",
-                          "--disable-Bsymbolic",
-                          "--enable-tools=yes",
-                          "--enable-pixbuf-loader=yes",
-                          "--enable-introspection=no"
+    args = ["--disable-dependency-tracking",
+            "--prefix=#{prefix}",
+            "--disable-Bsymbolic",
+            "--enable-tools=yes",
+            "--enable-pixbuf-loader=yes",
+            "--enable-introspection=no"]
+
+    args << "--enable-svgz" if build.with? 'libgsf'
+
+    system "./configure", *args
     system "make install"
   end
 end