Implemented SimpleBuilder and updated Environment#add_builder method to support it

lib/rscons/builders/simple_builder.rb

@@ -0,0 +1,37 @@
+module Rscons
+  module Builders
+    # A Generic builder class whose name and operation is defined at
+    # instantiation.
+    class SimpleBuilder < Builder
+      # The name of this builder when registered in an environment
+      attr_reader :name
+
+      # Create a new builder with the given name and action.
+      #
+      # @param name  [String,Symbol] The name of the builder when registered.
+      # @param block [Block]
+      #   The action to perform when the builder is processed. The provided
+      #   block must return the target file on success or false on failure.
+      #   The provided block should have the same signature as {Builder#run}.
+      def initialize(name, &block)
+        @name  = name.to_s
+        @block = block
+      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)
+        @block.call(target, sources, cache, env, vars)
+      end
+    end
+  end
+end
+