Update porters handbook to sync to new autotools world order:

• remove warning that the versions cannot be used for cross-development outside of the ports tree, this is no longer true

• add reference to the 'autotools' USE_AUTOTOOLS construct (for ports) and ports/devel/autotools to bring in all available versions of autotools

• remove table mapping old constructs to USE_AUTOTOOLS stanzas, since they haven't been present for a long time.

Reviewed by: pav