found a workaround supported by Arduino bugfix #182 (arduino/arduino-builder#182 (comment))

Author: simonrupf

libraries/MonSens/MonSens_ESP8266.h

@@ -30,6 +30,11 @@
 #ifndef MONSENS_ESP8266_H
 #define MONSENS_ESP8266_H
 
+// needs to be set before including the ESP libraries
+#ifdef MONSENS_ESP8266_VCC_H
+  ADC_MODE(ADC_VCC);
+#endif
+
 #include <MonSens.h>
 #include <ESP8266WiFi.h>
 

libraries/MonSens/MonSens_ESP8266_Vcc.h

@@ -31,10 +31,6 @@
 
 #include <MonSens.h>
 
-/* as of Arduino IDE 1.6.8, the below sequence doesn't work:
-ADC_MODE(ADC_VCC);
-*/
-
 /**
  * MonSens implementation for a Photoresistor
  */