There appears to be a failure in the Makefile build logic. Steps to reproduce:

1. Make a fresh build using "make clean && make"
2. run "touch Modules/_io/_iomodule.h"
3. Run "make" again.

Observed result: No related files get built.
Expected result: The directly including compilation units Modules/_io/{_iomodule,iobase,fileio,bufferedio,textio,bytesio,stringio}.o should be built. The compilation units which depend on these (e.g. Modules/textio.o) should also be built.

The problem appears to be related to Modules/makesetup, which adds rules to tbe Makefile. An example of a rule is

Modules/textio.o: $(srcdir)/Modules/_io/textio.c; $(CC) $(PY_BUILTIN_MODULE_CFLAGS)  -DPy_BUILD_CORE_BUILTIN -I$(srcdir)/Include/internal -I$(srcdir)/Modules/_io -c $(srcdir)/Modules/_io/textio.c -o Modules/textio.o

The problem here looks like the lack of a dependency between Modules/_io/textio.o and Modules/_io/textio.o - there is only a dependency on Modules/_io/textio.c, which wouldn't be rebuilt on a touch of Modules/_io/_iomodule.h.