autoreconf: require word boundaries when grepping for macros (#111271)

autoreconf: require word boundaries when grepping for macros (#111271)

autoreconf’s first pass over configure.ac is effectively a grep
operation for a number of macros that almost always appear directly
in that file, if they’re going to appear at all.  All but one of the
regular expressions made no attempt to check for word boundaries, and
the one that did was doing it incorrectly (using ^ rather than \b),
leading to issue #111271.

Fix all the regexps to surround each identifier with \b…\b and not
to use ^.

Reported by Kip, originally as gettext bug #52424.

* bin/autoreconf.in (autoreconf_current_directory): In the first pass,
  require each of the macros we look for to be a complete word.