Bug#883392: libc6-dev: break versions of libperl5.26 that require xlocale.h

[Niko Tyni <ntyni@debian.org>]

Package: libc6-dev
Version: 2.26-0experimental1 
x-debbugs-cc: perl@packages.debian.org

As noted in
  https://sourceware.org/glibc/wiki/Release/2.26#Removal_of_.27xlocale.h.27
glibc 2.26 dropped xlocale.h, which was never intended to be a public
header file.

The Perl build time configuration script looks for xlocale.h. If found,
'#include xlocale.h' gets inserted into perl.h in the libperl5.26 binary
package. This is a public header for building Perl extensions or embedding
a Perl interpreter. I suspect the inclusion comes from the BSD world,
where xlocale.h provides an interface for "thread-safe extended locale
support" (see https://www.freebsd.org/cgi/man.cgi?query=xlocale&sektion=3 )

This means that while including xlocale.h has been harmless on glibc
until now, with 2.26 it breaks building packages that include perl.h
(see #882978). I fixed this in perl_5.26.1-3 by explicitly disabling the
xlocale.h probing, but partial upgrades are still potentially affected.

Please add a Breaks: libperl5.26 (<< 5.26.1-3) into libc6-dev to fix this.

Thanks for your work,
-- 
Niko Tyni   ntyni@debian.org