wscript:
Add option `--example-tools` to allow optional build of executables and
libraries provided by jack-example-tools (defaults to True).
It can be used as `--example-tools=yes|no` or `--no-example-tools` or
plain `--example-tools`.
Based on the new option, set the configuration environment variable
`HAVE_JACK_EXAMPLE_TOOLS` which is used throughout the configuration
and build steps to include the `example-clients`, `tools` and `tests`
subdirectories.

man/wscript:
Provide the `HAVE_JACK_EXAMPLE_TOOLS` build environment variable to the
`fill_template` script as second argument.
Remove unused imports.

man/fill_template:
Change the behavior of the script to only create all man pages if the
second argument to the script is `True` (the jack-example-tools are
being built), else only create the man page for jackd.