Proper cont_t name for core 2.4.2 and higher

code/espurna/utils.ino

@@ -6,11 +6,31 @@ Copyright (C) 2017-2018 by Xose Pérez <xose dot perez at gmail dot com>
 
 */
 
+// Core version 2.4.2 and higher changed the cont_t structure to a pointer:
+// https://github.com/esp8266/Arduino/commit/5d5ea92a4d004ab009d5f642629946a0cb8893dd#diff-3fa12668b289ccb95b7ab334833a4ba8L35
+#if defined(ARDUINO_ESP8266_RELEASE_2_3_0) \
+    || defined(ARDUINO_ESP8266_RELEASE_2_4_0) \
+    || defined(ARDUINO_ESP8266_RELEASE_2_4_1)
 extern "C" {
     #include <cont.h>
     extern cont_t g_cont;
+
+}
+
+unsigned int getFreeStack() {
+    return cont_get_free_stack(&g_cont);
+}
+#else
+extern "C" {
+    #include <cont.h>
+    extern cont_t* g_pcont;
 }
 
+unsigned int getFreeStack() {
+    return cont_get_free_stack(g_pcont);
+}
+#endif // defined(ARDUINO_ESP8266_RELEASE_2_3_0/2_4_0/2_4_1)
+
 #include <Ticker.h>
 Ticker _defer_reset;
 
@@ -81,10 +101,6 @@ unsigned int getUsedHeap() {
     return getInitialFreeHeap() - getFreeHeap();
 }
 
-unsigned int getFreeStack() {
-    return cont_get_free_stack(&g_cont);
-}
-
 String getEspurnaModules() {
     return FPSTR(espurna_modules);
 }