fix circular build dependency detection logic

spec/build_tests_spec.rb

@@ -735,14 +735,14 @@ EOF
   it "raises an error for a circular dependency" do
     test_dir("simple")
     result = run_test(rsconsfile: "error_circular_dependency.rb")
-    expect(result.stderr).to match /Circular build dependency for (foo|bar|baz)/
+    expect(result.stderr).to match /Possible circular dependency for (foo|bar|baz)/
     expect(result.status).to_not eq 0
   end
 
   it "raises an error for a circular dependency where a build target contains itself in its source list" do
     test_dir("simple")
     result = run_test(rsconsfile: "error_circular_dependency2.rb")
-    expect(result.stderr).to match /Circular build dependency for foo/
+    expect(result.stderr).to match /Possible circular dependency for foo/
     expect(result.status).to_not eq 0
   end