Update dependency on gnat_util

[dalybrown]

To build gnatcov from source, we require gnat_util:

gnatcoverage/tools/gnatcov/README.md

Line 85 in 29f3005

4. If your GNAT toolchain does not come with a `gnat_util` library

.

However, this (as far as I know) is not distributed with the FSF version of gnat and AdaCore has stopped releasing the community edition. There is a patch for an older version of gnat_util on the alire project in github, but that seems like an unsustainable workaround. Is there a way to build gnatcov without depending on gnat_util? Thanks!

[andrewathalye]

gnat_util can be built (manually) from FSF gnat sources, but this isn’t ideal of course.

[Jugst3r]

Note that currently, Alire users are expected to use the prebuilt version of gnatcov. This is not ideal, but we have not found a way to inline just yet the dependency to gnat_util, hopefully next year.