Build Swig bindings for the MLT framework.

MLT is included with Slackware, but without bindings.
It is safe to install this package in addition to what Slackware
already provides.
This package does NOT conflict with the default MLT package, and
instead installs bindings as additions to what's already included.

If you upgrade the Slackware MLT package in the future, though,
you'll probably need to upgrade these bindings.
Alternately, you can rebuild the Slackware MLT package with bindings
enabled, in which case you would remove this package.

Despite CMake options suggesting that there are more, the bindings
currently available are:

* PERL
* PHP
* PYTHON
* RUBY

By default, all are built.
Disable a binding by setting an environment variable before building.
Either "no" or "off" are valid values.
For example, to exclude Python and Ruby bindings:

    # PYTHON=no RUBY=off sh ./mlt-bindings.SlackBuild