Add AC_PROG_EGREP to AU_DEFUN for AC_HEADER_STDC (#110215)

Add AC_PROG_EGREP to AU_DEFUN for AC_HEADER_STDC (#110215)

AC_HEADER_STDC used to use AC_EGREP_CPP, and therefore had the side
effect of AC_REQUIRE([AC_PROG_EGREP]).  In 2.70 AC_HEADER_STDC is an
AU_DEFUN and, before this change, the replacement didn’t invoke
AC_PROG_EGREP, which broke configure scripts that assumed $EGREP would
be set.  Problem reported by Ross Burton.

* lib/autoconf/headers.m4 (AU::AC_HEADER_STDC): Also invoke AC_PROG_EGREP.