fixed compilation for c++14

CMakeLists.txt

 cmake_minimum_required(VERSION 3.1)
 project(cartopy)
-set(CMAKE_CXX_STANDARD 17)
+set(CMAKE_CXX_STANDARD 14)
 find_package(pybind11 REQUIRED)
 find_package(cartographer REQUIRED)
 pybind11_add_module(cartopy cartopy.cpp)

cartopy.cpp

@@ -7,10 +7,10 @@
 #include <string>
 #include <type_traits>
 #define PROJECT_NAME "cartopy"
-namespace pybind11::detail {
+namespace pybind11 { namespace detail {
 template <typename T>
 struct type_caster<
-    T, std::enable_if_t<std::is_base_of_v<google::protobuf::Message, T>>> {
+    T, typename std::enable_if<std::is_base_of<google::protobuf::Message, T>::value>::type> {
   PYBIND11_TYPE_CASTER(T, _("google.protobuf.message.Message"));
   bool load(handle src, bool) { return false; }
   static handle cast(T src, return_value_policy policy,
@@ -31,6 +31,7 @@ struct type_caster<
   }
 };
 
+}
 } // namespace pybind11::detail
 
 PYBIND11_MODULE(cartopy, m) {