Make GNUmakefile work with in-place builds.
* configure.ac (AC_CONFIG_LINKS): Use m4_PACKAGE_VERSION to work
with older autoconf. Use shell variable to bypass older automake
distclean rules.
* Makefile.am (distclean-local): Remove GNUmakefile link in VPATH
builds, since we bypassed automake.
* GNUmakefile: Collapse redundant lines.
(_is-dist-target): 'distclean' is not a dist target.
* Makefile.maint (vc-diff-check): Support out-of-tree VPATH build.
Reported by Ralf Wildenhues.
Signed-off-by: Eric Blake <ebb9@byu.net>