Fun. The rwildcard function here

    rwildcard = $(foreach d,$(wildcard $1*),$(call rwildcard,$d/,$2) $(filter $2,$d))

is very similar to one I wrote in 2011 (https://blog.jgc.org/2011/07/gnu-make-recursive-wildcard-fun...):

    rwildcard=$(foreach d,$(wildcard $1*),$(call rwildcard,$d/,$2) $(filter $(subst *,%,$2),$d))

The only difference is that I wanted to be able to use * in the pattern rather than % (hence the subst).

    $(call rwildcard,,*.c)

vs.

    $(call rwildcard,,%.c)