imlib2: make X11 dependency optional.

References Homebrew/homebrew#15325.
Closes Homebrew/homebrew#15390.

Formula/imlib2.rb

@@ -5,15 +5,23 @@ class Imlib2 < Formula
   url 'http://downloads.sourceforge.net/project/enlightenment/imlib2-src/1.4.5/imlib2-1.4.5.tar.bz2'
   sha1 'af86a2c38f4bc3806db57e64e74dc9814ad474a0'
 
+  option "with-x", "Build with X support"
+
+  depends_on :freetype
+  depends_on :libpng => :recommended
+  depends_on :x11 if MacOS::X11.installed? or build.include? "with-x"
   depends_on 'pkg-config' => :build
   depends_on 'jpeg' => :recommended
-  depends_on :x11
 
   def install
-    system "./configure", "--disable-dependency-tracking",
-                          "--prefix=#{prefix}",
-                          "--enable-amd64=no",
-                          "--with-x"
+    args = %W[
+      --disable-dependency-tracking
+      --prefix=#{prefix}
+      --enable-amd64=no
+    ]
+    args << "--without-x" unless build.include? "with-x"
+
+    system "./configure", *args
     system "make install"
   end