cmLocalGenerator: Add AppendFlags overload with backtraces

Source/cmLocalGenerator.cxx

@@ -2138,6 +2138,14 @@ void cmLocalGenerator::AppendFlags(std::string& flags,
   }
 }
 
+void cmLocalGenerator::AppendFlags(
+  std::string& flags, const std::vector<BT<std::string>>& newFlags) const
+{
+  for (BT<std::string> const& flag : newFlags) {
+    this->AppendFlags(flags, flag.Value);
+  }
+}
+
 void cmLocalGenerator::AppendFlagEscape(std::string& flags,
                                         const std::string& rawFlag) const
 {

Source/cmLocalGenerator.h

@@ -121,6 +121,8 @@ public:
   //! Append flags to a string.
   virtual void AppendFlags(std::string& flags,
                            const std::string& newFlags) const;
+  virtual void AppendFlags(std::string& flags,
+                           const std::vector<BT<std::string>>& newFlags) const;
   virtual void AppendFlagEscape(std::string& flags,
                                 const std::string& rawFlag) const;
   void AddPchDependencies(cmGeneratorTarget* target,

Source/cmLocalUnixMakefileGenerator3.h

@@ -90,6 +90,7 @@ public:
   // append flags to a string
   void AppendFlags(std::string& flags,
                    const std::string& newFlags) const override;
+  using cmLocalCommonGenerator::AppendFlags;
 
   // append an echo command
   enum EchoColor