Make the example code buildable again
Remove unused-but-set-variables
Fix message to match failure
Conditionally build the example code and wireshark dissector

To activate, `BUILD_EXTRAS=1 ./build.sh`.
Pass `./build.sh` command-line arguments on to `cmake`

This makes it easier to do, for example, a debug type build.  Simply
add `-DCMAKE_BUILD_TYPE=Debug` to the `./build.sh` command-line and
you're good to go.
Drop install prefix for the extras

They aren't really supposed to be installed (at this point at least)
and a value of `/usr` gets in the way of package management tools.