[Debian GNUstep maintainers] Bug#646840: fails to build from source twice in a row

[Peter Eisentraut]

Package: gnustep-base
Version: 1.22.1-1
Severity: important

When building this package twice in a row, the second build fails thus:

 dpkg-source -i -I -b gnustep-base-1.22.1
dpkg-source: info: using source format `1.0'
dpkg-source: info: building gnustep-base using existing gnustep-base_1.22.1.orig.tar.gz
dpkg-source: info: building gnustep-base in gnustep-base_1.22.1-1.diff.gz
dpkg-source: error: cannot represent change to gnustep-base-1.22.1/Tests/base/NSException/core.basic.825: binary file contents changed
dpkg-source: error: cannot represent change to gnustep-base-1.22.1/Tests/base/NSTask/core.NSZombie.6814: binary file contents changed
dpkg-source: warning: the diff modifies the following upstream files: 
 Documentation/manual/manual.fns
dpkg-source: info: use the '3.0 (quilt)' format to have separate and documented changes to upstream files, see dpkg-source(1)
dpkg-source: unrepresentable changes to source
dpkg-buildpackage: error: dpkg-source -i -I -b gnustep-base-1.22.1 gave error exit status 1

Are these core files expected test output, or should I analyze further
why I get them?