configure failed since cannot find stdlib.h on Yocto Poky Linux

congfigure kea with --boost_include_path may cause stdlib.h cannot be found

it is caused by below commit: https://github.com/isc-projects/kea/commit/98e64a69d5271db2a6f4944b7c377ee6488a1a42

Change of ax_boost_for_kea.m4: - BOOST_INCLUDES="-I${boost_include_path}" + BOOST_INCLUDES="-isystem ${boost_include_path}"

and -isystem could cause this problem, refer: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70129 and https://stackoverflow.com/questions/37218953/isystem-on-a-system-include-directory-causes-errors