GHS: Shorten long object paths with duplicate source names

Detect when the resulting object path is too long and compute an
alternative name using a hash.

Source/cmLocalGenerator.h

@@ -404,4 +404,10 @@ private:
   void ComputeObjectMaxPath();
 };
 
+#if defined(CMAKE_BUILD_WITH_CMAKE)
+bool cmLocalGeneratorCheckObjectName(std::string &objName,
+                                     std::string::size_type dir_len,
+                                     std::string::size_type max_total_len);
+#endif
+
 #endif