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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: Example Ada calling Gnu Scientific Library (GSL)
Date: Mon, 10 Apr 2017 11:47:16 +0200
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Date: 2017-04-10T11:47:16+02:00
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On 10/04/2017 10:53, hreba wrote:

> I put this in all my 3 project files, called gprbuild with the -f option
> with all 3 project files - nothing new.

Because it has nothing to do with the linker package. Packages do not 
cross project boundary. See Linker_Options if you want to pass library 
switches.

> Is there no example with another thick binding to a library with C
> interface (and which is less huge than GtkAda)?

library project GSL is
    for Externally_Built use "true";
    for Source_Files use ();
    for Library_Dir use "/usr/lib"; -- Or as appropriate
    for Library_Name use "gsl";
    for Library_Kind use "relocatable"; -- Assuming gsl.so or gsl.dll
end GSL;

with "gsl.gpr";
library project GSL_Thin_Bindings is
    ...

You also seem confuse project to build and project to use a library. 
They are not same! For a bindings library you need both.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de