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/Makefile.am b/Makefile.am
index 723c9ed..fe94ea2 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -3,7 +3,7 @@
 SUBDIRS = include src doc tests contrib
 EXTRA_DIST = osmoappdesc.py README.md
 
-DISTCHECK_CONFIGURE_FLAGS = \
+AM_DISTCHECK_CONFIGURE_FLAGS = \
 	--with-systemdsystemunitdir=$$dc_install_base/$(systemdsystemunitdir)
 
 @RELMAKE@