GenexEval: Add abstracted access to link interface for a target.

This can be extended with special handling for static libraries
so that we can process the link dependencies separately from the
usage dependencies.

Source/cmGeneratorExpressionEvaluator.cxx

@@ -900,13 +900,14 @@ static const struct TargetPropertyNode : public cmGeneratorExpressionNode
     if (std::find_if(transBegin, transEnd,
               TransitiveWhitelistCompare(propertyName)) != transEnd)
       {
-      const cmTarget::LinkInterface *iface = target->GetLinkInterface(
+
-                                                    context->Config,
+      std::vector<std::string> libs;
-                                                    headTarget);
+      target->GetTransitivePropertyLinkLibraries(context->Config,
-      if(iface)
+                                                 headTarget, libs);
+      if (!libs.empty())
         {
         linkedTargetsContent =
-                  getLinkedTargetsContent(iface->Libraries, target,
+                  getLinkedTargetsContent(libs, target,
                                           headTarget,
                                           context, &dagChecker,
                                           interfacePropertyName);

Source/cmTarget.cxx

@@ -6001,6 +6001,22 @@ cmTarget::LinkInterface const* cmTarget::GetLinkInterface(const char* config,
   return i->second.Exists? &i->second : 0;
 }
 
+//----------------------------------------------------------------------------
+void cmTarget::GetTransitivePropertyLinkLibraries(
+                                      const char* config,
+                                      cmTarget *headTarget,
+                                      std::vector<std::string> &libs)
+{
+  cmTarget::LinkInterface const* iface = this->GetLinkInterface(config,
+                                                                headTarget);
+  if (!iface)
+    {
+    return;
+    }
+
+  libs = iface->Libraries;
+}
+
 //----------------------------------------------------------------------------
 bool cmTarget::ComputeLinkInterface(const char* config, LinkInterface& iface,
                                     cmTarget *headTarget)

Source/cmTarget.h

@@ -276,6 +276,9 @@ public:
       if the target cannot be linked.  */
   LinkInterface const* GetLinkInterface(const char* config,
                                         cmTarget *headTarget);
+  void GetTransitivePropertyLinkLibraries(const char* config,
+                                        cmTarget *headTarget,
+                                        std::vector<std::string> &libs);
 
   /** The link implementation specifies the direct library
       dependencies needed by the object files of the target.  */