build: Use git-version-gen intead of version.sh
* version.sh: Remove.
* autogen.sh: Import git-version-gen from Gnulib.
* Makefile.am (EXTRA_DIST): Add $(top_srcdir)/.version.
(BUILT_SOURCES): Likewise.
($(top_srcdir)/.version): New rule.
(dist-hook): New rule.
* configure.ac: Use git-version-gen in AC_INIT.
* gettext-runtime/configure.ac: Likewise.
* gettext-tools/configure.ac: Likewise.
* gettext-tools/examples/configure.ac: Likewise.
* gettext-runtime/man/Makefile.am: Refer to .version instead of
version.sh.
* gettext-tools/doc/Makefile.am: Likewise.
* gettext-tools/man/Makefile.am: Likewise.
