Cleanup/deprecation

[jklmnn]

With the update to GNAT 12 I noticed that there are more and more files that change in newer GNAT versions. With the latest version the pragma Compiler_Unit_Warning is no longer available which initially made the build fail on multiple occasions. I quick fixed that now by ignoring unknown pragmas with -gnatwG.

While trying to fix this I noticed that we have no proper solution for different versions. We have several copies of a-tags.ads, one for each compiler. All but one of them are the exact same file.

I see two options now:

• Fix the build system to handle these things better (and move the runtime to alire in that process)
• Discontinue the runtime

Currently I tend to favor the second option. Apart from posix none of the other platforms has been properly tested in years and as far as I can tell nobody is using this runtime.

@treiher @senier what's your opinion on this?

[senier]

@jklmnn I agree with you - we should deprecate it. It's not actively used and seems to become a maintenance burden with no benefit.