prj.gpr is a project using extends all feature.

It is based on:
GPRbuild and GPR Companion Tools User’s Guide
 2. GNAT Project Manager
  2.6. Project Extension
   2.6.1. Importing and Project Extension

see https://docs.adacore.com/gprbuild-docs/html/gprbuild_ug/gnat_project_manager.html#importing-and-project-extension

This project generates obj_main_ext/main executable

$ gprls -P prj.gpr  -U
extends_all/obj_main_ext/b.o
   B
      OK extends_all/src_b/b.adb
extends_all/obj_c_ext/c.o
   C
      OK extends_all/src_c/c.adb
extends_all/obj_a_ext/foo_ext.o
   FOO
      OK extends_all/src_a_ext/foo_ext.adb
extends_all/obj_main_ext/main.o
   MAIN
      OK extends_all/src_main/main.adb
