Add gtk-doc as a build dependency for libgpod and run gtkdocize during the build process to ensure proper documentation generation.

Formula/libgpod.rb

@@ -9,6 +9,7 @@ class Libgpod < Formula
   depends_on "automake" => :build
   depends_on "libtool" => :build
   depends_on "pkg-config" => :build
+  depends_on "gtk-doc" => :build
 
   depends_on "glib"
   depends_on "gdk-pixbuf"
@@ -20,6 +21,10 @@ class Libgpod < Formula
     # Upstream's autogen.sh is very old and may hardcode ancient automake checks
     # (e.g. looking for automake-1.7). Using autoreconf is the standard Homebrew
     # way and works with modern autotools.
+    #
+    # libgpod's build system expects gtk-doc's makefile snippet to exist (gtk-doc.make),
+    # which is normally provided by running gtkdocize.
+    system "gtkdocize", "--copy"
     system "autoreconf", "-fiv"
 
     system "./configure", *std_configure_args,