how to tell gnatmake to send executables to a different directory when compiling multi source?

["Nasser M. Abbasi" <nma@12000.org>]

I have  --+---src/main1.adb, main2.adb
           |
           +---obj/
           |
           +---bin/

In src/ I have few Ada main programs. I can build all and tell gnatmake
to send the *.o and *.ali to the obj/ directory using -D switch.

But not able to find an option to tell it send the generated
executable (main1.exe and main2.exe) to bin/  directory.

Otherwise, it will put them in src/

For example, when in src/ directory, I type

       gnatmake -D ../obj  *.adb

But I can't write something like

       gnatmake -D ../obj  -o../bin/  *.adb

Since it is a multi-file gnatmake call:

"gnatmake: cannot specify a single executable for several mains"
(it is actually think ../bin/ above is a name of a 'file')

If I want to use gnatmake from the command line, I would have to write

  gnatmake -D ../obj  -o../bin/main1.exe  main1.adb
  gnatmake -D ../obj  -o../bin/main2.exe  main2.adb

I looked at all options here

http://gcc.gnu.org/onlinedocs/gnat_ugn_unw/Switches-for-gnatmake.html#Switches-for-gnatmake

May be I missed it. Any one knows a trick to do this?

ps. I can do the above using a Makefile, using a rule, so
I do not have to type the above gnatmake command for each file ofcourse,
as Makefile will do it for me, using wildcard.

I am just interested to see if it is possible using a switch, that is all.

thanks
--Nasser
ps. I know gpr is supposed to be used for Ada, may be later when
I learn, but for now, I am using gnatmake.