Disable systemd support for "make distcheck"

"make distcheck" sets prefix to a user-owned directory, compiles code and
runs "make install". That fails if systemd files need to be installed, as
their location if not relative to the prefix.

Makefile.am

@@ -1,4 +1,5 @@
 ACLOCAL_AMFLAGS = -I m4
+AM_DISTCHECK_CONFIGURE_FLAGS = --without-systemdsystemunitdir
 
 EXTRA_DIST = bootstrap COPYING design.txt faq-compile.txt faq-general.txt file-loc.txt install.txt prog_std.txt readme.txt