emerge: enable --dynamic-deps=y by default once again (bug 646458)

There's been a lot of pushback involving the --dynamic-deps=n default.
What we really need is a tool to apply dependency changes in-place,
without the need for a rebuild.

Reverts: 2905e1c2c28d ("Disable dynamic-deps by default")
Bug: https://bugs.gentoo.org/646458
4 files changed