I found answer finally.
It isn't necessary to set environment variables. The .gpr files for the libraries are located in /usr/lib/gnat. You need to write such .gpr file for your project:
with "<name of lib's .gpr file related to /usr/lib/gnat";
project SUFO is
for Source_Dirs use ("src");
for Object_Dir use "obj";
for Exec_Dir use "bin";
for Main use ("src/main.adb");
end SUFO;
And run gprbuild
command in project directory, where you've placed your .gpr file.