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: Iaac6d41ef1cc2d5449edf870f635b28b045dc7db
diff --git a/Makefile.am b/Makefile.am
index 1dd1ff4..3fcbc70 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -5,7 +5,7 @@
 BUILT_SOURCES = $(top_srcdir)/.version
 EXTRA_DIST = git-version-gen .version
 
-DISTCHECK_CONFIGURE_FLAGS = \
+AM_DISTCHECK_CONFIGURE_FLAGS = \
 	--with-systemdsystemunitdir=$$dc_install_base/$(systemdsystemunitdir)