dist: distdir not unconditionally removed anymore for xz and lzip

dist: distdir not unconditionally removed anymore for xz and lzip

This change fixes automake bug#10444 a.k.a. bug#10448.

The 'dist-xz' and 'dist-lzip' recipes were erroneously using
'$(am__remove_distdir)' instead of '$(am__post_remove_distdir)'
to cleanup the '$(distdir)'; so a "make dist" issued in a package
using (say) gzip and xz compression would have failed to properly
created the gzip tarball, since the distdir was unconditionally
removed by "make dist-xz" upon its completion, instead of being
left populated for the following "make dist-gzip".

The problem with 'dist-xz' was introduced in the merge commit
`v1.11-1142-g47587d1', and the problem with 'dist-lzip' was
introduced in the merge commit `v1.11-1673-gc1b14e9'

* lib/am/distdir.am (dist-xz): Use '$(am__post_remove_distdir)',
not '$(am__remove_distdir)'.
(dist-lzip): Likewise.