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: I10d1eef9838c0b843a3a4103b7b03e8e9457b69e
diff --git a/contrib/jenkins.sh b/contrib/jenkins.sh
index 5f81978..ea1675c 100755
--- a/contrib/jenkins.sh
+++ b/contrib/jenkins.sh
@@ -77,7 +77,7 @@
 autoreconf --install --force
 ./configure $PCU_CONFIG
 $MAKE $PARALLEL_MAKE
-DISTCHECK_CONFIGURE_FLAGS="$PCU_CONFIG" AM_DISTCHECK_CONFIGURE_FLAGS="$PCU_CONFIG" \
+DISTCHECK_CONFIGURE_FLAGS="$PCU_CONFIG" \
   $MAKE distcheck \
   || cat-testlogs.sh