Bug#982887: libruby2.7: separate rbconfig.rb for cross building

[Helmut Grohne <helmut@subdivi.de>]

Package: libruby2.7
Version: 2.7.2-4
User: debian-cross@lists.debian.org
Usertags: ftcbfs
Control: affects -1 + src:ruby-json

ruby-json (and many others) fail to cross build from source in the same
way. dh_ruby runs ruby ... extconf.rb, which configures for the build
architecture and then some dependency (usually ruby/config.h) goes
missing as it is only installed for the host architecture.

As far as I can tell, the root cause is the extconf.rb invocation. For
cross compiling, one should pass -I /somepath to ruby such that
/somepath contains the host's rbconfig.rb. Unfortunately, we cannot just
pass -I /usr/lib/<triplet>/ruby/2.7.0 here, because doing so results in
ruby attempting to load foreign extension modules. We need a different
path here.

Let me give a little excursion into other ecosystems to give you a
better idea what is needed here:

Perl has a file that is similar to rbconfig.rb and it is simply called
Config.pm. It normally resides in /usr/lib/x86_64-linux-gnu/perl-base.
Using this path would have the same issue. Therefore, there also is
/usr/lib/x86_64-linux-gnu/perl/5.32.1 containing a symlink.

Python has a similar file _sysconfigdata.py. Rather than disambiguating
the containing directory, Python renames it and arrives at things like
_sysconfigdata__x86_64-linux-gnu.py. Rather than specifying a directory
to to search for modules, a separate environment variable can specify
its location.

The Ruby way is like Perl. I suggest copying this approach. To that end,
I request that ruby2.7-dev adds a new, architecture-dependent directory
that contains a symlink to the matching rbconfig.rb. Then dh_ruby can
pass that directory via -I and things should work.

Does that sound good to you? Do you have any preference on the naming?

No, this is not meant for bullseye.

Helmut