Do not rebuild targets when they are up to date

Store dependencies and file checksums in a cache file

spec/build_tests_spec.rb

@@ -17,6 +17,7 @@ describe Rscons do
 
   before do
     $stdout.stub(:puts) { nil }
+    Rscons::Cache.open.clear
   end
 
   ###########################################################################
@@ -31,4 +32,14 @@ describe Rscons do
       `./simple`.should =~ /This is a simple C program/
     end
   end
+
+  it 'does not rebuild the program if no sources changed' do
+    $stdout.should_receive(:puts).once.with('gcc -c -o simple.o simple.c')
+    $stdout.should_receive(:puts).once.with('gcc -o simple simple.o')
+    setup_testdir(['simple.c']) do
+      env = Rscons::Environment.new
+      env.Program('simple', 'simple.c')
+      env.Program('simple', 'simple.c')
+    end
+  end
 end