configure: build tablet-shell client conditionally

Do not build the tablet-shell client if --disable-tablet-shell is given.

Change --enable-tablet-shell to --disable-tablet-shell in ./configure
--help output, since it is enabled by default. Add a description.

Use proper quoting in the conditional.

Signed-off-by: Pekka Paalanen <ppaalanen@gmail.com>
2 files changed