cmGeneratorTarget: Refactor Fortran module directory lookup

Make `target_module_dir` and owned value so we can modify it.

Source/cmGeneratorTarget.cxx

@@ -3896,10 +3896,13 @@ std::string cmGeneratorTarget::GetFortranModuleDirectory() const
 std::string cmGeneratorTarget::CreateFortranModuleDirectory() const
 {
   std::string mod_dir;
-  const char* target_mod_dir = this->GetProperty("Fortran_MODULE_DIRECTORY");
+  std::string target_mod_dir;
+  if (const char* prop = this->GetProperty("Fortran_MODULE_DIRECTORY")) {
+    target_mod_dir = prop;
+  }
   const char* moddir_flag =
     this->Makefile->GetDefinition("CMAKE_Fortran_MODDIR_FLAG");
-  if (target_mod_dir && moddir_flag) {
+  if (!target_mod_dir.empty() && moddir_flag) {
     // Compute the full path to the module directory.
     if (cmSystemTools::FileIsFullPath(target_mod_dir)) {
       // Already a full path.