You could use: -gnateD on your command line or GPS options list

  -gnateD?  Define or redefine preprocessing symbol, e.g. -gnateDsym=val

Where use could set a variable to the architecture path you wish to use 
such as for a 32-bit arch

gnat make  -gnateDArchLib="/usr/lib/'  <filename>

     then for the 64 bit arch use

gnat make   -gnateDArchLib="/usr/lib64/'   <filename>


--
--  ArchLib is define in commnd line options. if not present an error 
--  will be generated.
--
   Libs_Path : String := ArchLib ;

   ...

   open ( File => LibFlile, 
          Name => Libs_Path & FileName, 
          Mode => In_File ) ;



In <75l5ceF18samiU1@mid.individual.net>, =?ISO-8859-1?Q?Bj=F6rn?= Persson <bjorn@xn--rombobjrn-67a.se> writes:
>When I write GNAT project files for shared libraries I find that I have to
>hardcode the paths to some architecture-specific directories. 32-bit
>libraries are in /usr/lib/ while 64-bit libraries are in /usr/lib64/, and I
>would like to have a single project file that works for both cases.
>
>The way to find out the architecture of a Unix-like system appears to be to
>invoke uname, but GNAT project files don't seem to have a shell-out
>feature. As far as I can see in the manual the only ways they can get input
>is by environment variables and command line parameters. I don't suppose
>Gnatmake provides some variable to project files that shows what the
>architecture is?
>
>I could define an environment variable in the systemwide profile, but that's
>not really a good solution. That variable would contain the host
>architecture but it's the target architecture I really need to test for. I
>also don't like polluting the environment.
>
>Does anyone have a better idea?
>
>-- 
>Bj�rn Persson
>PGP key A88682FD