putty: add optional dep on gtk+

Closes Homebrew/homebrew#17533.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>

Formula/putty.rb

@@ -5,18 +5,20 @@ class Putty < Formula
   url 'http://the.earth.li/~sgtatham/putty/0.62/putty-0.62.tar.gz'
   sha1 '5898438614117ee7e3704fc3f30a3c4bf2041380'
 
+  depends_on 'gtk+' => :optional
+
   def install
-    # use the unix build to make all PuTTY command line tools
     cd "unix" do
-      # disable GTK upon configure
-      system "./configure", "--prefix=#{prefix}",
-                            "--disable-gtktest",
-                            "--with-gtk-prefix=/dev/null"
-      system "make", "VER=-DRELEASE=#{version}", "all-cli"
-      # install manually
+      system "./configure", "--prefix=#{prefix}", "--disable-gtktest"
+      system "make", "VER=-DRELEASE=#{version}",
+                     (build.with?('gtk+') ? "all" : "all-cli")
+
+      bin.install %w{ putty puttytel pterm } if build.with? 'gtk+'
       bin.install %w{ plink pscp psftp puttygen }
     end
+
     cd "doc" do
+      man1.install %w{ putty.1 puttytel.1 pterm.1 } if build.with? 'gtk+'
       man1.install %w{ plink.1 pscp.1 psftp.1 puttygen.1 }
     end
   end