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: I6eb789b34523ed45a3626972f420d409fbab1597
diff --git a/Makefile.am b/Makefile.am
index 9f3644f..3f89896 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -19,7 +19,7 @@
 BUILT_SOURCES = $(top_srcdir)/.version
 EXTRA_DIST = git-version-gen osmoappdesc.py .version
 
-DISTCHECK_CONFIGURE_FLAGS = \
+AM_DISTCHECK_CONFIGURE_FLAGS = \
 	--with-systemdsystemunitdir=$$dc_install_base/$(systemdsystemunitdir)
 
 @RELMAKE@