dh_translations looks for po/Makefile in source tree, fails with CMake when built out-of-source

If a package doesn’t use intltool to manage its translations, the build_pot() function of /usr/bin/dh_translations looks for po/Makefile, and if it exists it executes "make $domain.pot". This works for packages that ship a Makefile in the po directory, but if the Makefile is generated at build time (e.g. when using CMake) in a separate build directory (which is usually the case when building packages), then dh_translations fails to find po/Makefile, even though it exists in the build directory.

Here is the relevant code:

    if ($use_intltool) {
        …
    } else {
        return unless -e 'po/Makefile';
    if ($domain) {
        # let the Makefile itself handle it
        @cmd=('make', $domain . '.pot');
    }

@cmd should be passed an additional '-C', $builddir.