version 2.15

* NEWS: Record release date.
maint: post-release administrivia

* NEWS: Add header line for next release.
* .prev-version: Record previous version.
* cfg.mk (old_NEWS_hash): Auto-update.
maint: avoid automake deprecation warning re ACLOCAL_AMFLAGS

* Makefile.am (ACLOCAL_AMFLAGS): Don't use this deprecated variable.
* configure.ac (AC_CONFIG_MACRO_DIRS): Use this instead.
(AUTOMAKE_OPTIONS): Require automake-1.12.
build: update gnulib submodule to latest

This pulls in a gnulib fix for maint.mk that ensures the procedure
described in README-release actually does what we want.  Before this
change, that procedure resulted in a grep-2.15 tarball that would
lead to a grep binary whose --version- reported version number was
2.14.51... rather than the expected 2.15.
maint: NEWS: document a release-related bug fix

* NEWS (Bug fixes): Add an entry for a fix pulled from gnulib.
grep: fix regression involving \s and \S

Commit v2.14-40-g01ec90b made \s and \S work with multi-byte
characters, but it made it so any use like \s*, \s+, \s?, \s{3}
would malfunction in a multi-byte locale.
* src/dfa.c (lex): Also reset laststart.
* tests/backslash-s-and-repetition-operators: New file.
* tests/Makefile.am (TESTS): Add it.
* NEWS (Bug fixes): Mention it.
* THANKS: Update.
Reported by Mirraz Mirraz in http://bugs.gnu.org/15773.
grep: fix regression with -P vs. invalid UTF-8 input

* src/pcresearch.c (Pexecute): Don't abort upon unexpected
PCRE-specific error code.  Explicitly handle PCRE_ERROR_BADUTF8,
and change the default to print a diagnostic including the unhandled
integer PCRE error code and exit with status 2.
* tests/pcre-invalid-utf8-input: New file.
* tests/Makefile.am (TESTS): Add it.
* NEWS (Bug fixes): Mention it.
* THANKS: Update.
Reported by Dave Reisner in http://bugs.gnu.org/15758.