zbar: x11 is optional

Closes Homebrew/homebrew#17653.

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

Formula/zbar.rb

@@ -5,19 +5,29 @@ class Zbar < Formula
   url 'http://downloads.sourceforge.net/project/zbar/zbar/0.10/zbar-0.10.tar.bz2'
   sha1 '273b47c26788faba4325baecc34063e27a012963'
 
-  depends_on :x11
+  depends_on :x11 => :optional
   depends_on 'pkg-config' => :build
   depends_on 'jpeg'
   depends_on 'imagemagick'
   depends_on 'ufraw'
 
   def install
-    system "./configure", "--disable-dependency-tracking",
-                          "--prefix=#{prefix}",
-                          "--without-python",
-                          "--without-qt",
-                          "--disable-video",
-                          "--without-gtk"
+    args = %W[
+      --disable-dependency-tracking
+      --prefix=#{prefix}
+      --without-python
+      --without-qt
+      --disable-video
+      --without-gtk
+    ]
+
+    if build.with? 'x11'
+      args << '--with-x'
+    else
+      args << '--without-x'
+    end
+
+    system "./configure", *args
     system "make install"
   end
 end