VS: Add internal API to get platform toolset as string

Source/cmGlobalVisualStudio10Generator.cxx

@@ -338,14 +338,21 @@ void cmGlobalVisualStudio10Generator::EnableLanguage(
 }
 
 const char* cmGlobalVisualStudio10Generator::GetPlatformToolset() const
+{
+  return this->GetPlatformToolsetString().c_str();
+}
+
+std::string const& cmGlobalVisualStudio10Generator::GetPlatformToolsetString()
+  const
 {
   if (!this->GeneratorToolset.empty()) {
-    return this->GeneratorToolset.c_str();
+    return this->GeneratorToolset;
   }
   if (!this->DefaultPlatformToolset.empty()) {
-    return this->DefaultPlatformToolset.c_str();
+    return this->DefaultPlatformToolset;
   }
-  return 0;
+  static std::string const empty;
+  return empty;
 }
 
 const char*

Source/cmGlobalVisualStudio10Generator.h

@@ -48,6 +48,7 @@ public:
 
   /** The toolset name for the target platform.  */
   const char* GetPlatformToolset() const;
+  std::string const& GetPlatformToolsetString() const;
 
   /** The toolset host architecture name (e.g. x64 for 64-bit host tools).  */
   const char* GetPlatformToolsetHostArchitecture() const;