fix Cache#up_to_date?() checking dependency subset - initial rebuilding only when necessary working

lib/rscons/cache.rb

@@ -60,7 +60,8 @@ module Rscons
       # command line used to build target must be identical
       return false unless @cache[target][:command] == command
       # all dependencies passed in must exist in cache (but cache may have more)
-      return false unless (Set.new(deps) - Set.new(@cache[target][:deps])).empty?
+      cached_deps = @cache[target][:deps].map { |dc| dc[:fname] }
+      return false unless (Set.new(deps) - Set.new(cached_deps)).empty?
       # all cached dependencies must have their checksums match
       @cache[target][:deps].map do |dep_cache|
         dep_cache[:checksum] == lookup_checksum(dep_cache[:fname])

spec/build_tests_spec.rb

@@ -79,7 +79,7 @@ describe Rscons do
     `./header`.should == "The value is 5\n"
   end
 
-  xit 'does not rebuild a C module when its dependencies have not changed' do
+  it 'does not rebuild a C module when its dependencies have not changed' do
     lines = test_dir('header')
     `./header`.should == "The value is 2\n"
     lines.should == [