Compiling C: main.c
avr-gcc -c -mmcu=atmega88 -I. -gdwarf-2 -DF_CPU=20000000UL -Os -funsigned-char -funsigned-bitfields -fpack-struct -fshort-enums -Wall -Wundef -Wa,-adhlns=obj/main.lst -I./include/ -std=gnu99 --combine -fwhole-program -Wundef -MD -MP -MF .dep/main.o.d main.c -o obj/main.o

Compiling C: spi.c
avr-gcc -c -mmcu=atmega88 -I. -gdwarf-2 -DF_CPU=20000000UL -Os -funsigned-char -funsigned-bitfields -fpack-struct -fshort-enums -Wall -Wundef -Wa,-adhlns=obj/spi.lst -I./include/ -std=gnu99 --combine -fwhole-program -Wundef -MD -MP -MF .dep/spi.o.d spi.c -o obj/spi.o

Linking: main.elf
avr-gcc -mmcu=atmega88 -I. -gdwarf-2 -DF_CPU=20000000UL -Os -funsigned-char -funsigned-bitfields -fpack-struct -fshort-enums -Wall -Wundef -Wa,-adhlns=obj/main.o -I./include/ -std=gnu99 --combine -fwhole-program -Wundef -MD -MP -MF .dep/main.elf.d obj/main.o obj/spi.o --output main.elf -Wl,-Map=main.map,--cref    -lm

obj/main.o: In function `main':
C:\Users\Christian\Desktop\ATmega\prog/main.c:5: undefined reference to `Test'
make: *** [main.elf] Error 1