Rework builder interface to only use #run method - close #91

The builder's #run method will be called repeatedly until it returns
true or false. The Builder#wait_for method can be used to cause a
builder to wait for a Thread, Command, or another Builder.

build_tests/two_sources/cache_strict_deps.rb

@@ -1,16 +1,16 @@
 class StrictBuilder < Rscons::Builder
   def run(options)
-    command = %W[gcc -o #{@target}] + @sources.sort
-    if @cache.up_to_date?(@target, command, @sources, @env, strict_deps: true)
-      @target
+    if @command
+      finalize_command
     else
-      ThreadedCommand.new(command, short_description: "#{name} #{@target}")
+      @command = %W[gcc -o #{@target}] + @sources.sort
+      if @cache.up_to_date?(@target, @command, @sources, @env, strict_deps: true)
+        true
+      else
+        register_command("#{name} #{@target}", @command)
+      end
     end
   end
-
-  def finalize(options)
-    standard_finalize(options)
-  end
 end
 
 build do