`make clean` rebuilds header dependency specifications

[SyntaxColoring]

When run on an already clean tree, make clean creates .d files only to immediately delete them.

% make clean
arm-none-eabi-gcc -MM -MF Source/Main.d -MT 'Source/Main.o Source/Main.d' Source/Main.c
arm-none-eabi-gcc -MM -MF Source/RCC.d -MT 'Source/RCC.o Source/RCC.d' Source/RCC.c
arm-none-eabi-gcc -MM -MF Source/LED.d -MT 'Source/LED.o Source/LED.d' Source/LED.c
rm -f Source/*.{o,d}
rm -f Image.elf
rm -f Image.bin
rm -f Blinky.o
rm -f Blinky.elf
rm -f Blinky.bin

This indicates I've fucked something up in the makefile. See https://stackoverflow.com/questions/12857020/gnu-make-clean-target-depends-on-includes.