add sanity checking to --with-iwyu

add some error checking so developers like me don't pass an empty
--with-iwyu option and get weird compile time error messages.

Change-Id: I142a0404db1bce905ce4d791f286d7bfc913078c
Reviewed-on: https://gerrit.libreoffice.org/12988
Reviewed-by: Noel Grandin <noelgrandin@gmail.com>
Tested-by: Noel Grandin <noelgrandin@gmail.com>