Define and use ARGV.build_devel?

Often it is useful to provide a development build in addition to the
stable release or HEAD download.

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

Library/Homebrew/extend/ARGV.rb

@@ -66,6 +66,10 @@ module HomebrewArgvExtension
     flag? '--HEAD'
   end
 
+  def build_devel?
+    include? '--devel'
+  end
+
   def build_universal?
     include? '--universal'
   end