cmLocalGenerator: Extract definition retrieval out of loop

Source/cmLocalGenerator.cxx

@@ -1176,12 +1176,13 @@ void cmLocalGenerator::GetTargetFlags(
           !(this->Makefile->IsOn("CYGWIN") || this->Makefile->IsOn("MINGW"))) {
         std::vector<cmSourceFile*> sources;
         target->GetSourceFiles(sources, buildType);
+        std::string defFlag =
+          this->Makefile->GetSafeDefinition("CMAKE_LINK_DEF_FILE_FLAG");
         for (std::vector<cmSourceFile*>::const_iterator i = sources.begin();
              i != sources.end(); ++i) {
           cmSourceFile* sf = *i;
           if (sf->GetExtension() == "def") {
-            linkFlags +=
-              this->Makefile->GetSafeDefinition("CMAKE_LINK_DEF_FILE_FLAG");
+            linkFlags += defFlag;
             linkFlags += this->ConvertToOutputFormat(
               cmSystemTools::CollapseFullPath(sf->GetFullPath()), SHELL);
             linkFlags += " ";