exports: accept a missing target if it is exported exactly once

If a target is exported, and a library it depends on is not part
of the same export set, before this patch cmake errored out.
With this patch, it now checks whether the missing target
is exported somewhere else exactly once, and accepts in this
case (because then it can determine the namespace for the
missing target and use this).

Alex

Source/cmExportBuildFileGenerator.h

@@ -46,7 +46,8 @@ protected:
                                            const char* config,
                                            std::string const& suffix);
   virtual void ComplainAboutMissingTarget(cmTarget* depender,
-                                          cmTarget* dependee);
+                                          cmTarget* dependee,
+                                          int occurrences);
 
   /** Fill in properties indicating built file locations.  */
   void SetImportLocationProperty(const char* config,