build: add the --disable-maintainer-mode option to ./configure

By passing --disable-maintainer-mode to ./configure the user can disable
autoconf build rules that would cause configure to be rebuilt and rerun.

Kind-of-requested-by: Mike Frysinger <vapier@gentoo.org>

configure.ac

@@ -20,6 +20,7 @@ AC_INIT([GNU nano], [6.2], [nano-devel@gnu.org], [nano])
 AC_CONFIG_SRCDIR([src/nano.c])
 AC_CANONICAL_HOST
 AM_INIT_AUTOMAKE([1.14])
+AM_MAINTAINER_MODE([enable])
 AC_CONFIG_HEADERS([config.h])
 
 dnl Make sure the ONCE macros are available.