build: use AM_DISTCHECK_CONFIGURE_FLAGS

According to
http://www.gnu.org/software/automake/manual/html_node/Checking-the-Distribution.html
the DISTCHECK_CONFIGURE_FLAGS is for the user, while
AM_DISTCHECK_CONFIGURE_FLAGS is the one to use in Makefile.am.

Make it so.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Boyan Ding <stu_dby@126.com>
Reviewed-by: Marek Chalupa <mchqwerty@gmail.com>

Makefile.am

@@ -8,7 +8,7 @@ module_LTLIBRARIES =
 noinst_LTLIBRARIES =
 BUILT_SOURCES =
 
-DISTCHECK_CONFIGURE_FLAGS = --disable-setuid-install
+AM_DISTCHECK_CONFIGURE_FLAGS = --disable-setuid-install
 
 EXTRA_DIST = weston.ini.in