[bug-cflow] question about automake recipe

[Britton Kerin]

The manual chapter 11 shows:

          program_CFLOW_INPUT=$(program_OBJECTS:.$(OBJEXT)=.c)
          program.cflow: program_CFLOW_INPUT cflow.rc Makefile
          CFLOWRC=path-to-your-cflow.rc \
          cflow -o$@ $(CFLOW_FLAGS) $(DEFS) \
                              $(DEFAULT_INCLUDES) $(INCLUDES) $(AM_CPPFLAGS) \
                     $(CPPFLAGS) \
                   $(program_CFLOW_INPUT)

I'm wondering why program_CFLOW_INPUT is computed this way, and if
it's ok to instead say:

           program_CFLOW_INPUT = $(filter %.c,$(program_SOURCES))

I guess there may be a reason, and I wonder if it's because sometimes
program_SOURCES
doesn't exist?  I very vaguely remember long ago reading in automake
manual an example
Makefile.am that had only program_OBJS, so mayge that's why?  But
nowadays the docs say to make program_SOURCES and maybe I remember
wrong?

Thanks,
Brittom