Conditionalize building makewhatis(1) for upgrades that need it.

Conditionalize building makewhatis(1) for upgrades that need it.

Remove building of the legacy makewhatis(1) since it was only needed for
6.0 upgrades.

On my 2.2 GHz system libsqlite3 takes 60-100 seconds to build, which due to
its serialized nature can hold up the build waiting on it to finish in
bootstrap-tools.

makewhatis(1) was only required to be a build tool to support upgrades
from 6.0 which was properly removed in r208324 but later reverted due to
installworld using it.  The installworld issue was fixed in r275622
when it was added to ITOOLS.  A BOOTSTRAPPING check was missed when
makewhatis(1) was replaced with mandoc in r283777.

Sponsored by:	EMC / Isilon Storage Division