Merge from 0.23.x branch: Prepare for 0.23.1 release.

Merge from 0.23.x branch: Prepare for 0.23.1 release.

* gettext-runtime/intl/libgnuintl.in.h (LIBINTL_VERSION): Bump.
* libtextstyle/version.sh: Update VERSION_NUMBER, RELEASE_DATE.
* gettext-tools/libgettextpo/gettext-po.in.h (LIBGETTEXTPO_VERSION): Bump.
* gettext-runtime/intl/Makefile.am (LTV_*): Bump to 12:2:4.
* libtextstyle/lib/Makefile.am (LTV_*): Bump to 2:3:2.
* gettext-tools/libgettextpo/Makefile.am (LTV_*): Bump to 5:12:5.
* gettext-tools/misc/autopoint.in: Likewise. Accept version 0.23.1.
* gettext-tools/configure.ac (ARCHIVE_VERSION): Bump to 0.23.1.