Add :when_xquartz_installed as a keg-only reason

Using :when_xquartz_installed will tell the keg-only machinery to activate
if XQuartz is installed.

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

Library/Homebrew/cmd/info.rb

@@ -63,7 +63,7 @@ module Homebrew extend self
     if f.keg_only?
       puts
       puts "This formula is keg-only."
-      puts f.keg_only?
+      puts f.keg_only_reason
       puts
     end
 

Library/Homebrew/formula.rb

@@ -174,7 +174,12 @@ class Formula
   # rarely, you don't want your library symlinked into the main prefix
   # see gettext.rb for an example
   def keg_only?
-    self.class.keg_only_reason || false
+    kor = self.class.keg_only_reason
+    not kor.nil? and kor.valid?
+  end
+
+  def keg_only_reason
+    self.class.keg_only_reason
   end
 
   def fails_with? cc

Library/Homebrew/formula_installer.rb

@@ -425,7 +425,7 @@ end
 class Formula
   def keg_only_text
     # Add indent into reason so undent won't truncate the beginnings of lines
-    reason = self.keg_only?.to_s.gsub(/[\n]/, "\n    ")
+    reason = self.keg_only_reason.to_s.gsub(/[\n]/, "\n    ")
     return <<-EOS.undent
     This formula is keg-only, so it was not symlinked into #{HOMEBREW_PREFIX}.
 

Library/Homebrew/formula_support.rb

@@ -136,19 +136,32 @@ class KegOnlyReason
   def initialize reason, explanation=nil
     @reason = reason
     @explanation = explanation
+    @valid = case @reason
+      when :when_xquartz_installed then MacOS::XQuartz.installed?
+      else true
+      end
+  end
+
+  def valid?
+    @valid
   end
 
   def to_s
-    if @reason == :provided_by_osx
-      <<-EOS.strip
-Mac OS X already provides this program and installing another version in
-parallel can cause all kinds of trouble.
+    case @reason
+    when :provided_by_osx then <<-EOS.undent
+      Mac OS X already provides this software and installing another version in
+      parallel can cause all kinds of trouble.
 
-#{@explanation}
-EOS
+      #{@explanation}
+      EOS
+    when :when_xquartz_installed then <<-EOS.undent
+      XQuartz provides this software.
+
+      #{@explanation}
+      EOS
     else
-      @reason.strip
-    end
+      @reason
+    end.strip
   end
 end