parse makefile depends generated by gcc

spec/build_tests_spec.rb

@@ -49,7 +49,7 @@ describe Rscons do
   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 -c -o simple.o -MMD -MF simple.mf simple.c")
     $stdout.should_receive(:puts).once.with('gcc -o simple simple.o')
     setup_testdir(['simple.c']) do
       env = Rscons::Environment.new