add Library default builder

lib/rscons/builders/library.rb

@@ -0,0 +1,33 @@
+require 'fileutils'
+
+module Rscons
+  # A default RScons builder that produces a static library archive.
+  class Library < Builder
+    def default_variables(env)
+      {
+        'AR' => 'ar',
+        'LIBSUFFIX' => '.a',
+        'ARFLAGS' => [],
+        'ARCOM' => ['$AR', 'rcs', '$ARFLAGS', '$TARGET', '$SOURCES']
+      }
+    end
+
+    def run(target, sources, cache, env, vars = {})
+      # build sources to linkable objects
+      objects = env.build_sources(sources, [env['OBJSUFFIX'], env['LIBSUFFIX']].flatten, cache, vars)
+      if objects
+        vars = vars.merge({
+          'TARGET' => target,
+          'SOURCES' => objects,
+        })
+        command = env.build_command(env['ARCOM'], vars)
+        unless cache.up_to_date?(target, command, objects)
+          FileUtils.rm_f(target)
+          return false unless env.execute("AR #{target}", command)
+          cache.register_build(target, command, objects)
+        end
+        target
+      end
+    end
+  end
+end