Fix tests and NoMethodError with no X11 installed.

Library/Homebrew/extend/ENV.rb

@@ -301,9 +301,10 @@ Please take one of the following actions:
     end
   end
 
-  def x11
+  def x11 silent=false
     unless MacOS::X11.installed?
-      opoo "You do not have X11 installed, this formula may not build."
+      opoo "You do not have X11 installed, this formula may not build." unless silent
+      return
     end
 
     # There are some config scripts here that should go in the PATH

Library/Homebrew/test/test_ENV.rb

@@ -15,7 +15,7 @@ class EnvironmentTests < Test::Unit::TestCase
     ENV.minimal_optimization
     ENV.no_optimization
     ENV.libxml2
-    ENV.x11
+    ENV.x11 true
     ENV.enable_warnings
     assert !ENV.cc.empty?
     assert !ENV.cxx.empty?