Make regex a define exclusive (#617)

* Change compiler error to runtime error.
Add compiler warining when a ASYNCWEBSERVER_REGEX function is used, but not enabled

* Update docs for usage in Arduino IDE

* Update docs for platform.local.txt

examples/regex_patterns/.test.build_flags

@@ -0,0 +1 @@
+-DASYNCWEBSERVER_REGEX=1

examples/regex_patterns/regex_patterns.ino

@@ -5,6 +5,18 @@
 //  * handle missing pages / 404s
 //
 
+// Add buildflag ASYNCWEBSERVER_REGEX to enable the regex support
+
+// For platformio: platformio.ini:
+//  build_flags = 
+//      -DASYNCWEBSERVER_REGEX
+
+// For arduino IDE: create/update platform.local.txt
+// Windows: C:\Users\(username)\AppData\Local\Arduino15\packages\espxxxx\hardware\espxxxx\{version}\platform.local.txt
+// Linux: ~/.arduino15/packages/espxxxx/hardware/espxxxx/{version}/platform.local.txt
+//
+// compiler.cpp.extra_flags=-DASYNCWEBSERVER_REGEX=1
+
 #include <Arduino.h>
 #ifdef ESP32
 #include <WiFi.h>