limit build to C++17 capable toolchains only

Author: vortigont

src/AsyncWSocket.cpp

@@ -3,6 +3,8 @@
 
 // A new experimental implementation of Async WebSockets client/server
 
+// We target C++17 capable toolchain
+#if __cplusplus >= 201703L
 #include "AsyncWSocket.h"
 #include "literals.h"
 
@@ -939,3 +941,4 @@ void WSocketServerWorker::_taskRunner(){
   vTaskDelete(NULL);
 }
 
+#endif  // __cplusplus >= 201703L

src/AsyncWSocket.h

@@ -3,7 +3,7 @@
 
 // A new experimental implementation of Async WebSockets client/server
 
-
+#if __cplusplus >= 201703L
 #pragma once
 
 #include "AsyncWebSocket.h"
@@ -945,3 +945,7 @@ class WSocketServerWorker : public WSocketServer {
   void _taskRunner();
 
 };
+
+#else  // __cplusplus >= 201703L
+#warning "WSocket requires C++17, won't build"
+#endif // __cplusplus >= 201703L