raise error when circular dependencies are found - close #38

build_tests/simple/error_circular_dependency.rb

@@ -0,0 +1,5 @@
+Rscons::Environment.new do |env|
+  env.Command("foo", "bar")
+  env.Command("bar", "baz")
+  env.Command("baz", "foo")
+end

build_tests/simple/error_circular_dependency2.rb

@@ -0,0 +1,3 @@
+Rscons::Environment.new do |env|
+  env.Command("foo", "foo")
+end