[matheusmoreira]

The find program might not be available, right? The developer might be building on Windows or something. Also, spawning new processes is an expensive operation so doing it from inside GNU Make might be faster.

Okay, I just wanted to see if I could do it in pure GNU Make.

  true := T
  not = $(if $(1),,$(true))

  directory? = $(if $(1),$(wildcard $(addsuffix /.,$(1))))
  file? = $(and $(wildcard $(1)),$(call not,$(call directory?,$(1))))

  glob = $(sort $(wildcard $(or $(1),*)))
  glob.directory = $(call glob,$(addsuffix /$(or $(2),*),$(or $(1),.)))

  recurse = $(foreach x,$(3),$(if $(call $(2),$(x)),$(x),$(x) $(call recurse,$(1),$(2),$(call $(1),$(x)))))

  file_system.traverse = $(call recurse,glob.directory,file?,$(or $(1),.))

  find = $(strip $(foreach entry,$(call file_system.traverse,$(1)),$(if $(call $(or $(2),true),$(entry)),$(entry))))

  sources := $(call find,src,file?)

Yes.

Then I discovered GNU Make supports C extensions. It will even automatically build them due to the way the include keyword works. It might actually be easier to just make a plugin with all the functionality I want...