Added a command builder to the list of default builders

lib/rscons/builders/command.rb

@@ -0,0 +1,30 @@
+module Rscons
+  module Builders
+    # Execute a command that will produce the given target based on the given
+    # sources.
+    #
+    # Example::
+    #   env.Command("docs.html", "docs.md",
+    #       CMD => ['pandoc', '-fmarkdown', '-thtml', '-o${_TARGET}', '${_SOURCES}'])
+    class Command < Builder
+      # 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)
+        vars = vars.merge({
+          "_TARGET" => target,
+          "_SOURCES" => sources,
+        })
+        command = env.build_command("${CMD}", vars)
+        standard_build("CMD #{target}", target, command, sources, env, cache)
+      end
+    end
+  end
+end