cmOrderDirectories: Factor out directory comparison

Create an `IsSameDirectory` helper method.

Source/cmOrderDirectories.cxx

@@ -73,10 +73,8 @@ public:
       {
       // Check if this directory conflicts with the entry.
       std::string const& dir = this->OD->OriginalDirectories[i];
-      if(dir != this->Directory &&
+      if (!this->OD->IsSameDirectory(dir, this->Directory) &&
-         cmSystemTools::GetRealPath(dir) !=
+          this->FindConflict(dir))
-         cmSystemTools::GetRealPath(this->Directory) &&
-         this->FindConflict(dir))
         {
         // The library will be found in this directory but this is not
         // the directory named for it.  Add an entry to make sure the
@@ -639,3 +637,10 @@ void cmOrderDirectories::DiagnoseCycle()
     ->IssueMessage(cmake::WARNING, e.str(),
                    this->Target->Target->GetBacktrace());
 }
+
+bool cmOrderDirectories::IsSameDirectory(std::string const& l,
+                                         std::string const& r)
+{
+  return (l == r ||
+          cmSystemTools::GetRealPath(l) == cmSystemTools::GetRealPath(r));
+}

Source/cmOrderDirectories.h

@@ -80,6 +80,9 @@ private:
   struct ConflictList: public std::vector<ConflictPair> {};
   std::vector<ConflictList> ConflictGraph;
 
+  // Compare directories after resolving symlinks.
+  bool IsSameDirectory(std::string const& l, std::string const& r);
+
   friend class cmOrderDirectoriesConstraint;
   friend class cmOrderDirectoriesConstraintLibrary;
 };