Add CONFIGURE_OUTSOURCE support

If CONFIGURE_OUTSOURCE is defined HAS_CONFIGURE invokes configure script
and builds the port out of source tree, under ${WRKDIR}/.build by default.
The feature is inspired by USES=cmake:outsource and USES=qmake:outsource.

PR: 208294
Exp-run by: antoine
Approved by: portmgr (mat)
Differential Revision: https://reviews.freebsd.org/D4157