I just tried compiling adafruitio_secure_esp8266 on my machine with the WeMos D1 Mini Lite selected as the board, did not run into the compilation error you did...
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[SNIP]
Using library ESP8266WiFi at version 1.0 in folder: /Users/brentrubell/Library/Arduino15/packages/esp8266/hardware/esp8266/2.5.2/libraries/ESP8266WiFi
Using library Adafruit_MQTT_Library at version 1.0.3 in folder: /Users/brentrubell/Documents/Arduino/libraries/Adafruit_MQTT_Library
/Users/brentrubell/Library/Arduino15/packages/esp8266/tools/xtensa-lx106-elf-gcc/2.5.0-3-20ed2b9/bin/xtensa-lx106-elf-size -A /var/folders/k6/g066g1tn567fn8mj2p42x4240000gn/T/arduino_build_351330/adafruitio_secure_esp8266.ino.elf
Sketch uses 385760 bytes (37%) of program storage space. Maximum is 1023984 bytes.
Global variables use 29100 bytes (35%) of dynamic memory, leaving 52820 bytes for local variables. Maximum is 81920 bytes.
Two things stick out to me, though:
Multiple libraries were found for "ESP8266WiFi.h"
and
Multiple libraries were found for "Adafruit_MQTT.h"
This might be causing the error. Close the Arduino IDE. Then, navigate to /Users/Deneb/Documents/Arduino/hardware/ and delete the esp8266com folder(s, there may be more than one). Then navigate to /Users/Deneb/Documents/Arduino/libraries/ and delete all instances of Adafruit_MQTT_Library.
Open Arduino and install the ESP8266 board support package again, along with Adafruit MQTT.