* Don't look for gprbuild from system provider.
Because the manifest currently asks for Alire to attempt to download
gprbuild from the system package manager, alr <anything> spends
several fruitless seconds trying to find gprbuild.
* index/gp/gprbuild/gprbuild-external.toml: remove the 'system' external.
* macOS distributions don't provide gprbuild, others do.
* index/gp/gprbuild/gprbuild-external.toml: the "system" external isn't
available on macOS, so specify it explicitly for the other operating
systems.
* GNAT 11.2.2
The Linux versions are built on ubuntu 18 instead of 20 for the .1
releases.
* GPRbuild 21.0.1
The Linux versions are built on ubuntu 18 instead of 20 for the .1
releases.
* GNATcov 21.0.1
The Linux versions are built on ubuntu 18 instead of 20 for the .1
releases.
* Change binary crate version from X.Y.0-1 to X.Y.1
All those project don't have a patch version so we are using it for
package version.
* gnat_native-11.2.1.toml: typo in URL
César Sagaert
César Sagaert
Alejandro R Mosteo
Fabien Chouteau
Maxim Reznik
Rowan Walshe
Simon Wright