Make cycles in target properties ignored, not an error.

Constructs such as these are an error as they are direct self-references:

 set_property(TARGET foo APPEND PROPERTY
   INCLUDE_DIRECTORIES $<TARGET_PROPERTY:foo,INCLUDE_DIRECTORIES>)
 set_property(TARGET foo APPEND PROPERTY
   INCLUDE_DIRECTORIES $<TARGET_PROPERTY:INCLUDE_DIRECTORIES>)

However, this is an indirect self-reference in a cycle, and not an error:

 set_property(TARGET foo APPEND PROPERTY
   INCLUDE_DIRECTORIES $<TARGET_PROPERTY:bar,INCLUDE_DIRECTORIES>)
 set_property(TARGET bar APPEND PROPERTY
   INCLUDE_DIRECTORIES $<TARGET_PROPERTY:foo,INCLUDE_DIRECTORIES>)

1 files changed, 7 insertions, 1 deletions

diff --git a/Source/cmGeneratorExpressionEvaluator.cxx b/Source/cmGeneratorExpressionEvaluator.cxx
index 8b8b860..4b44ebe 100644
--- a/Source/cmGeneratorExpressionEvaluator.cxx
+++ b/Source/cmGeneratorExpressionEvaluator.cxx
@@ -381,10 +381,16 @@ static const struct TargetPropertyNode : public cmGeneratorExpressionNode
                                                content,
                                                dagCheckerParent);
 
-    if (!dagChecker.check())
+    switch (dagChecker.check())
       {
+    case cmGeneratorExpressionDAGChecker::SELF_REFERENCE:
       dagChecker.reportError(context, content->GetOriginalExpression());
       return std::string();
+    case cmGeneratorExpressionDAGChecker::CYCLIC_REFERENCE:
+      // No error. We just skip cyclic references.
+      return std::string();
+    case cmGeneratorExpressionDAGChecker::DAG:
+      break;
       }
 
     const char *prop = target->GetProperty(propertyName.c_str());