compiling C files with gnat project files

["evangeli@cnam.fr" <evangeli@cnam.fr>]

Hello,
I am trying to use gnat project files in my project which contains
some Ada and C files.
I have the following tree :

--==========
src/
   helena.gpr
src/utils/
   sizeof.h
   sizeof.c
src/progs/
   helena_main.adb
src/obj/
--==========

src/progs/helena_main.adb must be compiled and linked with sizeof.o
I want to compile src/progs/helena_main.adb into executable helena and
put it into src/../bin/
so here is the helena.gpr I made:

--==========
project Helena is
   for Object_Dir use "obj";
   for Exec_Dir use "../bin";
   for Source_Dirs use ("utils", "progs");
   for Languages use ("Ada", "C");
   for Main use ("helena_main.adb");
   package Compiler is
      for Default_Switches ("Ada") use
        ("-O3", "-gnat05", "-gnata", "-gnatyM79", "-gnatwvrmkc");
      for Default_Switches ("C") use ("-O3");
   end Compiler;
   package Linker is
      for Default_Switches ("Ada") use ("obj/sizeof.o");
      for Default_Switches ("C") use ();
   end Linker;
   package Builder is
      for Executable ("helena_main.adb") use "helena";
   end Builder;
end Helena;
--==========

when I launch the following command:
> gnatmake -P helena.gpr
the file sizeof.c in src/utils is not compiled.
however, if I compile sizeof.c manually and put sizeof.o in src/obj
the compilation works fine.
I looked at
http://www.adacore.com/wp-content/files/auto_update/gnat-unw-docs/html/gnat_ugn_12.html
but i did not find help on how to do this.

thanks for any help
Sami