How to configure GNAT GPL on x86-64 Linux for ARM ELF development

[Adam Jensen <hanzer@riseup.net>]

Prologue:

I would like to tinker with Ada and Spark embedded real-time software 
development. I have these two books:

https://www.amazon.com/Concurrent-Real-Time-Programming-Alan-Burns/dp/
0521866979/
https://www.amazon.com/Analysable-Real-Time-Systems-Programmed-Ada/dp/
1530265509/

And these two books are on the way:

https://www.amazon.com/Building-Parallel-Embedded-Real-Time-Applications/
dp/0521197163/
https://www.amazon.com/Building-High-Integrity-Applications-SPARK/dp/
1107656842/

To start, I have a Nucleo-144 board with an STM32F429ZIT6 MCU. 

https://www.digikey.com/product-detail/en/stmicroelectronics/NUCLEO-
F429ZI/497-16280-ND/5806777

I have installed Adacore's Gnat GPL and SPARK Discovery on Ubuntu 18.04 
LTS in: 

$HOME/.local/gnat

And I've installed gnat-gpl-2017-arm-elf-linux-bin.tar.gz in:

$HOME/.local/gnat-arm

I am following this tutorial:

http://www.inspirel.com/articles/Ada_On_Cortex.html
-------------------------------------------------------------------------

The first example in that tutorial is:

#program.ads
package Program is

   procedure Run;
   pragma Export (C, Run, "run");

end Program;

# program.adb
package body Program is

   procedure Run is
   begin
      loop
         null;
      end loop;
   end Run;

end Program;

# flash.ld
OUTPUT_FORMAT("elf32-littlearm")
OUTPUT_ARCH(arm)

SECTIONS
{
    .vectors 0x08000000 :
    {
        LONG(0x20010000)
        LONG(run + 1)
        FILL(0)
    }

    .text 0x08000200 :
    {
        *(.text)
    }
}
-------------------------------------------------------------------------

Given:

PATH="$HOME/.local/gnat-arm/bin:$PATH"; export PATH

The tutorial suggests that maybe this (below) might produce a binary 
suitable to be loaded onto the MCU:

arm-eabi-gcc -c -mcpu=cortex-m4 -mthumb program.adb
arm-eabi-ld -T flash.ld -o program.elf program.o 
arm-eabi-objcopy -O binary program.elf program.bin

This is what actually happens:

$ arm-eabi-gcc -c -mcpu=cortex-m4 -mthumb program.adb
fatal error, run-time library not installed correctly
cannot locate file system.ads
compilation abandoned

I guess that the LD_LIBRARY_PATH and GPR_PROJECT_PATH environment 
variables should be set but I don't yet understand enough to make 
reasonable guesses. 

Any advice on how to proceed would be very much appreciated!