Add DTrace and Makefile dependency includes.

Sample file mpq.d by Georg Lukas; license GPL 2.
Sample file counts.d by Kate Turner; public domain.
Sample file javascript-race.d by unknown; license MPL 1.1/GPL 2.0/LGPL 2.1.
Sample file probes.d by momjian; license TBD.

lib/linguist/heuristics.rb

@@ -229,5 +229,15 @@ module Linguist
         Language["Text"]
       end
     end
+
+    disambiguate "D", "DTrace", "Makefile" do |data|
+      if /^module /.match(data)
+        Language["D"]
+      elsif /^((dtrace:::)?BEGIN|provider |#pragma (D (option|attributes)|ident)\s)/.match(data)
+        Language["DTrace"]
+      elsif /(\/.*:( .* \\)$| : \\$|^ : |: \\$)/.match(data)
+        Language["Makefile"]
+      end
+    end
   end
 end