cmake: disable tests and man generation if perl/nroff not found

Fixes https://github.com/curl/curl/issues/1500
Reported-by: Jay Satiro

Fixes https://github.com/curl/curl/pull/1662


Assisted-by: Tom Seddon
Assisted-by:  <dpull@users.noreply.github.com>
Assisted-by:  <elelel@users.noreply.github.com>

Closes https://github.com/curl/curl/pull/1924