Introduce version restrictions for Autotools.

configure.ac

@@ -6,11 +6,12 @@ AC_CONFIG_AUX_DIR([config])
 AC_CONFIG_SRCDIR([include/mtime_calendar.h])
 AC_CONFIG_HEADERS([config/config.h])
 
-AM_INIT_AUTOMAKE([foreign])
+AM_INIT_AUTOMAKE([1.16.1 foreign])
 AM_MAINTAINER_MODE([disable])
 AM_EXTRA_RECURSIVE_TARGETS([doc install-doc man install-man])
 m4_ifdef([AM_SILENT_RULES], [AM_SILENT_RULES([yes])])
 
+LT_PREREQ([2.4.6])dnl
 LT_INIT([pic-only])
 dnl Clean up after Libtool checks for C compiler:
 rm -f a.out