shadow: fix build break during cross compiling

This patch provides for shadow cross compile build break,
AC_RUN_IFELSE configure method replaced by AC_CHECK_SIZEOF
built-in size check.

patch ref: http://lists.alioth.debian.org/pipermail/
pkg-shadow-devel/2014-August/010399.html

Signed-off-by: Shrikant Bobade <Shrikant_Bobade@mentor.com>