If you enable verbose output during compile, it will tell you
Using board 'promicro' from platform in folder: C:\Users\sterretje\AppData\Local\Arduino15\packages\SparkFun\hardware\avr\1.1.12
Using core 'arduino' from platform in folder: C:\Users\sterretje\AppData\Local\Arduino15\packages\arduino\hardware\avr\1.8.3
Detecting libraries used...
"C:\\Users\\sterretje\\AppData\\Local\\Arduino15\\packages\\arduino\\tools\\avr-gcc\\7.3.0-atmel3.6.1-arduino7/bin/avr-g++" -c -g -Os -w -std=gnu++11 -fpermissive -fno-exceptions -ffunction-sections -fdata-sections -fno-threadsafe-statics -flto -w -x c++ -E -CC -mmcu=atmega32u4 -DF_CPU=8000000L -DARDUINO=10607 -DARDUINO_AVR_PROMICRO -DARDUINO_ARCH_AVR -DUSB_VID=0x1b4f -DUSB_PID=0x9204 "-DUSB_MANUFACTURER=\"Unknown\"" "-DUSB_PRODUCT=\"SparkFun Pro Micro\"" "-IC:\\Users\\sterretje\\AppData\\Local\\Arduino15\\packages\\arduino\\hardware\\avr\\1.8.3\\cores\\arduino" "-IC:\\Users\\sterretje\\AppData\\Local\\Arduino15\\packages\\SparkFun\\hardware\\avr\\1.1.12\\variants\\promicro" "C:\\Users\\sterretje\\AppData\\Local\\Temp\\arduino-sketch-CE22C7A4E842C457B61D122D4E5BACD3\\sketch\\random_techniques.ino.cpp" -o nul
Alternatives for sterretje.h: []
ResolveLibrary(sterretje.h)
-> candidates: []
Compilation error: Error: 2 UNKNOWN: exit status 1
Note that this was done with beta 7.