brew doctor - X11 check

Tweak wording in X11 check and use the same X11 detection function
as ENV.x11; also move that function into utils.

Library/Homebrew/brew.h.rb

@@ -619,7 +619,3 @@ def llvm_build
     $1.to_i
   end
 end
-
-def x11_installed?
-  Pathname.new('/usr/X11/lib/libpng.dylib').exist?
-end

Library/Homebrew/brew_doctor.rb

@@ -54,10 +54,11 @@ def check_for_stray_dylibs
 end
 
 def check_for_x11
-  unless File.exists? '/usr/X11/lib/libpng.dylib'
+  unless x11_installed?
+    opoo "X11 not installed."
     puts <<-EOS.undent
       You don't have X11 installed as part of your Xcode installation.
-      This isn't required for all formula, but is expected by some.
+      This isn't required for all formulae, but is expected by some.
 
     EOS
   end

Library/Homebrew/utils.rb

@@ -277,3 +277,7 @@ def dump_build_env env
     puts "#{k}: #{value}" if value
   end
 end
+
+def x11_installed?
+  Pathname.new('/usr/X11/lib/libpng.dylib').exist?
+end