Workaround for intltool vs. automake issue during distcheck

intltool and automake 1.15 currently cause a problem during make distcheck.
This is a no-op workaround for intltool-update to make that succeed again.
See https://github.com/systemd/systemd/commit/06528e8fdd and
https://bugs.launchpad.net/intltool/+bug/1117944 for more information.

Thanks to Daniel Mack for pointing this out during Chaos Communication Camp.

configure.ac

@@ -212,6 +212,11 @@ AC_SUBST(GETTEXT_PACKAGE)
 AC_DEFINE_UNQUOTED(GETTEXT_PACKAGE, "$GETTEXT_PACKAGE",
                    [The prefix for our gettext translation domains.])
 
+# work around intltool-update issues during 'make distcheck'
+AS_IF([test "x$0" != "x./configure"], [
+        AC_SUBST([INTLTOOL_UPDATE], [/bin/true])
+])
+
 # Determine a C compiler to use
 AC_PROG_CC
 AM_PROG_CC_C_O