build: Use AM_DISTCHECK_CONFIGURE_FLAGS
build: Use AM_DISTCHECK_CONFIGURE_FLAGS

DISTCHECK_CONFIGURE_FLAGS (no prefix) is for the user to override the
distcheck configure flags. The AM_-prefixed version is for build systems
to use.

https://www.gnu.org/software/automake/manual/html_node/Checking-the-Distribution.html

pwithnall/walbottle