Ada file depends on C file

[mockturtle <framefritti@gmail.com>]

Dear all,
I have a question that I am afraid is awfully trivial, but I spent
half a morning without finding any answer.

My setup: I am working with GPS and I have an Ada procedure (say
Interface_to_Addr in foo.adb) that needs to call a small C function
(char *interface_to_ip (char*)) defined in interface_to_ip.c.  So, in
my Ada code I wrote

   function interface_to_ip(interf : chars_ptr)  return chars_ptr;
   pragma Import(C, interface_to_ip);

Everything compiles fine, but when it is time to link, the linker
complains (correctly) that it cannot find "interface_to_ip".  The
problem is that the compiler does not know that interface_to_ip is in
interface_to_ip.c, or, in other words, it does not know that foo.adb
depends on interface_to_ip.c.

Currently I solved by adding the line

    pragma Linker_Options("../../interface_to_ip.o");

and by compiling "by hand" interface_to_ip.c to interface_to_ip.o, but
this solution is *UGLY*.

Everything would be fine if I could tell to the compiler that foo.adb
depends on interface_to_ip.c, but I do not know how to do it.  I
looked under the GPS "Project" menu, browsed the GNAT user & reference
manuals (although not in the deepest detail) and googled for a while,
but I could not find an answer.

Thank you in advance for your help.