Ticket #3029: configure.ac: warning about unknown '--with-' / '--enable-' options.

As we use 'AC_CONFIG_SUBDIRS' macro for samba sources option checking is
disabled for us.

Before the patch:

    $ ./configure --with-i-have-misspelled-editor-for-edit
    ...
    # no warnings

The patch enables warnings back only for top-level ./configure:

    $ ./configure --with-i-have-misspelled-editor-for-edit
    ...
    configure: WARNING: unrecognized options: --with-i-have-misspelled-editor-for-edit

Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>
Merge branch '3029_warn_about_unknown_switches'

* 3029_warn_about_unknown_switches:
  Ticket #3029: configure.ac: warning about unknown '--with-' / '--enable-' options.