How can I install the ZFP (Zero Foot Print) RTS (Run Time System) for AVR with the Alire package manager for Ada?

My project file, I think correctly, contains:

project Avr is
   for Runtime("Ada") use "zfp";
   for Target use "avr-elf";
end Avr;

alire.toml hopefully correction contains:

[[depends-on]]
gnat_avr_elf = ">=11.2.4"

Unfortunately, when running alr build, I get:

gprconfig: can't find a toolchain for the following configuration:
gprconfig: language 'ada', target 'avr-elf', runtime 'zfp'

I found documentation for programming AVR with Ada, but this assumes that I build the tool-chain myself and not have a package manager at least providing the GNU tool-chain.

The same applies to Programming Arduino with Ada.