Path prefix to src and targets

Hi all,

I would like to realize the following:
How can I "automatically" prefix the current subdirectory in the included
Makefile relative to the "main" Makefile for all targets and source files?

directory tree:

Makefile
subdir_a/
   Makefile
   file.c
subdir_b/
   subdir_c/
     Makefile
     file.c
   Makefile
   otherfile.c

When calling the "main" Makefile, the subdirectory-Makefiles get recursively
include'd into the mail Makefile and thus the targets from each sub-Makefile.
Problem here:
file.c will exist twice (and thus file.o as target) and the target rules might
miss the subdirectory-prefixes, so the build will not work properly.

Thanks.

Best regards,

Erik