build: keep --version strictly up to date

build: keep --version strictly up to date

Before this change, in development, gzip's --version output could lag
behind reality by a couple deltas or by a "-dirty" suffix.  That would
lead to spurious failure of the new --version-$VERSION PATH cross-check.
* Makefile.am (version.c, version.h): New rules.
(BUILT_SOURCES): Set/append.
(noinst_LIBRARIES, noinst_libver_a_SOURCES): Define.
(gzip_LDADD): Add libver.a.
(DISTCLEANFILES): Define.
* gzip.c (license): Use Version, not VERSION.