Add proper help for with-build-platform-configure-options

Change-Id: I9ebe739223f396f16f326c7fbad09c762030f796
Reviewed-on: https://gerrit.libreoffice.org/11406
Reviewed-by: David Tardon <dtardon@redhat.com>
Tested-by: David Tardon <dtardon@redhat.com>