debian/rules: ensure build is done with distro build flags
Set EXTRA_CFLAGS to CFLAGS as passed by debhelper. Use DEB_CFLAGS_MAINT_APPEND to honor existing build systemd defaults.
Set EXTRA_CFLAGS to CFLAGS as passed by debhelper. Use DEB_CFLAGS_MAINT_APPEND to honor existing build systemd defaults.