Declare __get_adc_mode and __get_rf_mode with C++ linkage

Workaround for #1759. Using extern "C" qualifiers in sketch breaks compilation in Arduino IDE 1.6.8.

Author: igrr

cores/esp8266/Esp.h

@@ -55,7 +55,8 @@ enum RFMode {
     RF_DISABLED = 4 // disable RF after deep-sleep wake up, just like modem sleep, there will be the smallest current.
 };
 
-#define RF_MODE(mode) extern "C" int __get_rf_mode() { return mode; }
+#define RF_MODE(mode) int __get_rf_mode() { return mode; }
+#define RF_PRE_INIT() void __run_user_rf_pre_init()
 
 // compatibility definitions
 #define WakeMode RFMode
@@ -71,7 +72,7 @@ enum ADCMode {
     ADC_VDD = 255
 };
 
-#define ADC_MODE(mode) extern "C" int __get_adc_mode(void) { return (int) (mode); }
+#define ADC_MODE(mode) int __get_adc_mode(void) { return (int) (mode); }
 
 typedef enum {
      FM_QIO = 0x00,

cores/esp8266/core_esp8266_phy.c

@@ -242,6 +242,14 @@ static const uint8_t ICACHE_FLASH_ATTR phy_init_data[128] =
     [114] = 2
 };
 
+// These functions will be overriden from C++ code.
+// Unfortunately, we can't use extern "C" because Arduino preprocessor
+// doesn't generate forward declarations for extern "C" functions correctly,
+// so we use mangled names here.
+#define __get_adc_mode _Z14__get_adc_modev
+#define __get_rf_mode _Z13__get_rf_modev
+#define __run_user_rf_pre_init _Z22__run_user_rf_pre_initv
+
 extern int __real_register_chipv6_phy(uint8_t* init_data);
 extern int __wrap_register_chipv6_phy(uint8_t* init_data) {
     if (init_data != NULL) {

tests/test_overrides/test_overrides.ino

@@ -0,0 +1,11 @@
+ADC_MODE(ADC_VCC);
+RF_MODE(RF_DISABLED);
+RF_PRE_INIT()
+{
+}
+
+void setup() {
+}
+
+void loop() {
+}