extract.? variables not set if extract phase is not run

[Mike Alexander]

The various extract.? variables which are set by (for example) 
use_bzip2 are only set if the extract phase is executed.  This means 
that ports like git-core which use these variables in other phases 
(destroot in this case) won't work right if the extract phase is run in 
one command and the other phase in another command.  I attached a patch 
to <http://trac.macports.org/ticket/29707> which fixes this.

          Mike