Address warnings on Clang 3.9 caused by having a static data member in a class template.

code/XGLLoader.cpp

@@ -83,8 +83,11 @@ struct free_it
 };
 
 namespace Assimp { // this has to be in here because LogFunctions is in ::Assimp
-template<> const std::string LogFunctions<XGLImporter>::log_prefix = "XGL: ";
-
+    template<> const char* LogFunctions<XGLImporter>::Prefix()
+    {
+        static auto prefix = "XGL: ";
+        return prefix;
+    }
 }
 
 static const aiImporterDesc desc = {