configure.ac: Support --disable-maintainer-mode

This

| allows you to choose whether the so called "rebuild rules" should be
| enabled or disabled.  With AM_MAINTAINER_MODE([enable]), they are
| enabled by default, otherwise they are disabled by default.  In the
| latter case, if you have AM_MAINTAINER_MODE in configure.ac, and run
| `./configure && make', then make will *never* attempt to rebuild
| configure, Makefile.ins, Lex or Yacc outputs, etc.  I.e., this
| disables build rules for files that are usually distributed and that
| users should normally not have to update.
|
| The user can override the default setting by passing either
| `--enable-maintainer-mode' or `--disable-maintainer-mode' to
| configure.
|
| People use AM_MAINTAINER_MODE either because they do not want their
| users (or themselves) annoyed by timestamps lossage (see CVS), or
| because they simply can't stand the rebuild rules and prefer running
| maintainer tools explicitly.

[ https://www.gnu.org/software/automake/manual/automake.html ]

configure.ac

@@ -6,6 +6,7 @@ AC_CONFIG_FILES([gl/Makefile
 	monitoring-plugins.spec])
 AC_CONFIG_AUX_DIR(build-aux)
 AM_INIT_AUTOMAKE([1.8.3])
+AM_MAINTAINER_MODE([enable])
 AC_CONFIG_HEADERS([config.h])
 AC_CANONICAL_HOST