Add feature to enable CORS during the build.

platformio.ini

@@ -14,7 +14,9 @@ framework = arduino
 ;upload_flags = --port=8266 --auth=esp-react
 ;upload_port = 192.168.0.6
 board_f_cpu = 160000000L
-build_flags= -D NO_GLOBAL_ARDUINOOTA
+build_flags=
+  -D NO_GLOBAL_ARDUINOOTA
+; -D ENABLE_CORS
 lib_deps =
   https://github.com/PaulStoffregen/Time
   https://github.com/gmag11/NtpClient

src/main.cpp

@@ -44,14 +44,23 @@ void setup() {
     server.serveStatic("/app/", SPIFFS, "/www/app/");
 
     // Serving all other get requests with "/www/index.htm"
+    // OPTIONS get a straight up 200 response
     server.onNotFound([](AsyncWebServerRequest *request) {
     	if (request->method() == HTTP_GET) {
         request->send(SPIFFS, "/www/index.html");
+      } else if (request->method() == HTTP_OPTIONS) {
+		    request->send(200);
       } else {
     		request->send(404);
     	}
     });
 
+    // Disable CORS if required
+    #if defined(ENABLE_CORS)
+    DefaultHeaders::Instance().addHeader("Access-Control-Allow-Origin", "*");
+    DefaultHeaders::Instance().addHeader("Access-Control-Allow-Headers", "*");
+    #endif
+
     server.begin();
 }