cmNinjaTargetGenerator: Factor out helper for forced response file check

Source/cmNinjaNormalTargetGenerator.cxx

@@ -698,9 +698,7 @@ void cmNinjaNormalTargetGenerator::WriteLinkStatement()
   cmGlobalNinjaGenerator& globalGen = *this->GetGlobalGenerator();
 
   int commandLineLengthLimit = 1;
-  const char* forceRspFile = "CMAKE_NINJA_FORCE_RESPONSE_FILE";
-  if (!mf->IsDefinitionSet(forceRspFile) &&
-      cmSystemTools::GetEnv(forceRspFile) == 0)
+  if (!this->ForceResponseFile())
     {
     commandLineLengthLimit = calculateCommandLineLengthLimit(
                 globalGen.GetRuleCmdLength(this->LanguageLinkerRule()));

Source/cmNinjaTargetGenerator.cxx

@@ -795,3 +795,10 @@ void cmNinjaTargetGenerator::addPoolNinjaVariable(
       vars["pool"] = pool;
       }
 }
+
+bool cmNinjaTargetGenerator::ForceResponseFile()
+{
+  static std::string const forceRspFile = "CMAKE_NINJA_FORCE_RESPONSE_FILE";
+  return (this->GetMakefile()->IsDefinitionSet(forceRspFile) ||
+          cmSystemTools::GetEnv(forceRspFile) != 0);
+}

Source/cmNinjaTargetGenerator.h

@@ -152,6 +152,8 @@ protected:
                             cmGeneratorTarget* target,
                             cmNinjaVars& vars);
 
+  bool ForceResponseFile();
+
 private:
   cmLocalNinjaGenerator* LocalGenerator;
   /// List of object files for this target.