Fix DISTCHECK_CONFIGURE_FLAGS override
Fix DISTCHECK_CONFIGURE_FLAGS override

Set AM_DISTCHECK_CONFIGURE_FLAGS in Makefile.am instead of
DISTCHECK_CONFIGURE_FLAGS. This is the recommended way from the
automake manual, as otherwise the flag can't be changed by the user
anymore.

Related: OS#3718
Change-Id: I332c94502cce0f3f11fe3f4d9f6c9918ff0c0263

osmocom/osmo-bts,osmocom/osmo-bts,osmocom/osmo-bts