build: add --without-swig to override swig check

build: add --without-swig to override swig check

Add the --without-swig option to disable the automatic swig detection
and disable the optional features which depend on swig.  This allows
builders to avoid swig even if present on the build system.

Also, add the --with-swig option to force the check and fail if not
detected.  This allows builders to declare the swig features are
mandatory.

The default continues to be to check for swig, and if present, build the
optional features which require swig.

To disable the automatic check for swig and disable the features which
depend on swig:

    ./configure --without-swig     # or --with-swig=no

To force the check and fail if swig is not present on the system:

    ./configure --with-swig        # or --with-swig=yes

If --with-swig is given and swig is not detected, then configure will
fail with the message:

    configure: error: swig requested but not found

The Perl 5 bindings for libuafs is the only feature which requires swig
at this time.

Change-Id: I0726658a9cc7b1b2a9d5e5d306adb6e36ad3c0f6
Reviewed-on: https://gerrit.openafs.org/12518
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>