net-im/prosody: Document SSL requirement and rework port makefile

This makefile had both <pre> and <options> but only the <options>
include was necesssary so the <pre> and <post> includes were remoed.
Most of the variables conventionally introduced before the <options>
include were wrapped and moved up to ensure they were defined when
needed. Finally SSL flags were added to support non-base SSL linkage.

Approved by: SSL blanket + general blankets