Bug#954681: [gnat-9] crash caused by SOURCE_DATE_EPOCH patch

[Nicolas Boulenguez <nicolas@debian.org>]

Package: src:gcc-9
Followup-For: Bug #954681
Control: reopen -1

Hello.

The initial symptom is cured, but the fix lets
debian/patches/ada-lib-info-source-date-epoch.diff modify
gcc/ada/libgnat/s-os_lib.ad[bs].

This invalidates the checksums embedded in the .ali files of
libgnat-9, causing a failure in all Ada compilations involving
packaged dependencies. Especially, all run time tests for packaged
libraries fail.

Ludovic suggests to revert the fix (commit
c1d276b0e2cb60140389aa4c23ac3addb7a77af3 )in the gcc-9 branch.

I suggest to cherry-pick the commit in the gcc-10 branch, so that it
is part of the future gnat-10 transition.

Matthias, do you agree with these two actions?

Then, we will search for a less intrusive work-around for this bug in
gcc-9.