Makefile: Move to AM_DISTCHECK_CONFIGURE_FLAGS

Since automake 1.11.2 it is recommended that packages
use AM_DISTCHECK_CONFIGURE_FLAGS instead of
DISTCHECK_CONFIGURE_FLAGS as the latter is intended
to be a user variable.

https://bugs.freedesktop.org/show_bug.cgi?id=95826

Makefile.am

@@ -19,7 +19,7 @@ SUBDIRS = \
 	docs \
 	$(NULL)
 
-DISTCHECK_CONFIGURE_FLAGS = \
+AM_DISTCHECK_CONFIGURE_FLAGS = \
 	--with-udev-base-dir="$$dc_install_base" \
 	--with-systemdsystemunitdir="$$dc_install_base/$(SYSTEMD_UNIT_DIR)" \
 	--enable-gtk-doc=yes \