build object files for sources

build_tests/c-simple/spec.rb

@@ -1,5 +1,6 @@
 it 'builds a C program with one source file' do
   env = Rscons::Environment.new
   env.Program('simple', 'simple.c')
+  File.exist?('simple.o').should be_true
   `./simple`.should =~ /This is a simple C program/
 end