* lib/autoconf/programs.m4 (AC_PROG_GREP): New macro to test for a

* lib/autoconf/programs.m4 (AC_PROG_GREP): New macro to test for a
grep or ggrep program in PATH that accepts as long lines as
possible.
* lib/autoconf/programs.m4 (_AC_PROG_GREP): Factor out the core of
AC_PROG_GREP.
(AC_PROG_EGREP, AC_PROG_FGREP): Use it to find best available
egrep and fgrep respectively if $GREP -E/-F don't work.
(_AC_PATH_PROG_FEATURE_CHECK): Factor out the common core of
_AC_PROG_GREP, and AC_PROG_SED.
(_AC_FEATURE_CHECK_LENGTH): New helper macro for finding the
longest input length accepted by a command.
(AC_PROG_SED): Use it.
* doc/autoconf.texi (Particular Programs): Document the changes.
* NEWS: Updated.