Merged rules

You can write rules multiple times:

 all: foo  # "all" depends on "foo"
 all: bar  # oh, all also depends on "bar"

This is useful for the standard *.d files GCC produces.

 foo.o: foo.c
   $(CC) -MD $< -o $@  # With -MD, GCC outputs foo.d which tells make the list of headers foo.c includes.
 -include *.d  # Include files GCC outputted, if any.