[coreboot] How the file, c_start.d is made?

[ali hagigat]

The only rule for building c_start.o from c_start.S is the following
in build/arch/i386/lib/c_start.d:
build/arch/i386/lib/c_start.o: src/arch/i386/lib/c_start.S \
/root/build/coreboot/coreboot-v4/build/config.h \
src/include/cpu/x86/post_code.h
The question is that what rule is making the file, c_start.d itself?