add a Builder#run method which raises an error unless overridden

lib/rscons/builder.rb

@@ -54,6 +54,20 @@ module Rscons
       false
     end
 
+    # Run the builder to produce a build target.
+    #
+    # @param target [String] Target file name.
+    # @param sources [Array<String>] Source file name(s).
+    # @param cache [Cache] The Cache object.
+    # @param env [Environment] The Environment executing the builder.
+    # @param vars [Hash,VarSet] Extra construction variables.
+    #
+    # @return [String,false]
+    #   Name of the target file on success or false on failure.
+    def run(target, sources, cache, env, vars)
+      raise "This method must be overridden in a subclass"
+    end
+
     # Check if the cache is up to date for the target and if not execute the
     # build command.
     #