Use AC_CONFIG_MACRO_DIR, not AC_CONFIG_MACRO_DIRS

AC_CONFIG_MACRO_DIRS requires a very recent Automake, much newer than the
declared version 1.6 (even 1.11 doesn't work). AC_CONFIG_MACRO_DIR is
defined by Autoconf starting with version 2.58, so it's within the
currently declared constraints.

configure.ac

@@ -4,7 +4,7 @@ AC_PREREQ(2.59)
 AC_INIT([xrdp], [0.9.0], [xrdp-devel@googlegroups.com])
 AC_CONFIG_HEADERS(config_ac.h:config_ac-h.in)
 AM_INIT_AUTOMAKE([1.6 foreign])
-AC_CONFIG_MACRO_DIRS([m4])
+AC_CONFIG_MACRO_DIR([m4])
 AC_PROG_CC
 AC_C_CONST
 AC_PROG_LIBTOOL