Introduce the new "scons" USES. The goal is to replace the old bsd.scons.mk
Instead of redifining the building target, let's just reuse the existing ones.
Bonus, MAKE_JOBS is now respected

Conflicts:
	CHANGES
Sometimes scons do expect DESTDIR being in lowercase
scons expect lots of variables to be set as arguments
Add USES=execinfo to handle dependency on libexecinfo avoiding pull a useless
libdependency on ports libexecinfo on freebsd 10+
Fix typo.

Sponsored by:	Absolight
import mongodb26 from 10.0@re
support old ports tree
support old ports tree

- remove LICENSE*, which is not supported
- use old LIB_DEPENDS format
fix do-build and do-install target