Tag Xcode and CLT requirements as build-time

This way they can be skipped when installing bottles.

Library/Homebrew/dependencies.rb

@@ -229,6 +229,7 @@ class Requirement
 
   def initialize(*tags)
     @tags = tags.flatten.compact
+    @tags << :build if self.class.build
   end
 
   # The message to show when the requirement is not met.
@@ -287,6 +288,10 @@ class Requirement
       val.nil? ? @fatal : @fatal = val
     end
 
+    def build(val=nil)
+      val.nil? ? @build : @build = val
+    end
+
     def satisfy(options={}, &block)
       @satisfied ||= Requirement::Satisfier.new(options, &block)
     end

Library/Homebrew/requirements.rb

@@ -204,6 +204,7 @@ end
 
 class XcodeDependency < Requirement
   fatal true
+  build true
 
   satisfy(:build_env => false) { MacOS::Xcode.installed? }
 
@@ -274,6 +275,7 @@ end
 
 class CLTDependency < Requirement
   fatal true
+  build true
 
   def satisfied?
     MacOS::CLT.installed?

Library/Homebrew/test/test_requirement.rb

@@ -87,4 +87,9 @@ class RequirementTests < Test::Unit::TestCase
 
     req.modify_build_environment
   end
+
+  def test_dsl_build
+    req = Class.new(Requirement) { build true }.new
+    assert req.build?
+  end
 end