Use serial tests option

We don't take any benefit from the parallel harness, since we have only
one test binary in each directory, and automake can't parallelize that.
But it was hiding test output, which was annoying.

Acked on the mailing list.

configure.ac

@@ -4,7 +4,7 @@
 AC_PREREQ([2.59])
 AC_INIT(bind10, 20121219, bind10-dev@isc.org)
 AC_CONFIG_SRCDIR(README)
-AM_INIT_AUTOMAKE([foreign])
+AM_INIT_AUTOMAKE([foreign serial-tests])
 m4_ifdef([AM_SILENT_RULES], [AM_SILENT_RULES([yes])])dnl be backward compatible
 AC_CONFIG_HEADERS([config.h])
 AC_CONFIG_MACRO_DIR([m4macros])