Add "depends_on :clt"

Library/Homebrew/dependencies.rb

@@ -86,6 +86,8 @@ private
       PostgresqlInstalled.new(tag)
     when :tex
       TeXInstalled.new(tag)
+    when :clt
+      CLTDependency.new(tag)
     else
       raise "Unsupported special dependency #{spec}"
     end

Library/Homebrew/requirements.rb

@@ -271,3 +271,18 @@ class TeXInstalled < Requirement
     EOS
   end
 end
+
+class CLTDependency < Requirement
+  fatal true
+
+  def satisfied?
+    MacOS::CLT.installed?
+  end
+
+  def message; <<-EOS.undent
+    The Command Line Tools for Xcode are required to compile this software.
+    The standalone package can be obtained from http://connect.apple.com,
+    or it can be installed via Xcode's preferences.
+    EOS
+  end
+end