Require bison 2.7 or newer for regenerating intl/plural.y

Require bison 2.7 or newer for regenerating intl/plural.y

The merge of the latest gettext code introduced changes to the yacc
parser source that are incompatible with versions of bison older
than 2.7. Add a configure check for the appropriate versions and
document the requirement in INSTALL.

ChangeLog:

2014-12-22  Will Newton  <will.newton@linaro.org>

	* manual/install.texi: Document that we require bison 2.7
	or above.
	* INSTALL: Regenerate.
	* configure.ac: Use AC_CHECK_PROG_VER instead of
	AC_PATH_PROG when checking for bison and check for
	version 2.7 or above.
	* configure: Regenerate.