[beta] backport #48181 and #48362#48793

Merged

bors merged 3 commits intorust-lang:betafrom

cuviper:beta-1.25-backports

Mar 9, 2018

Commits on Mar 6, 2018

incr.comp.: Run cache directory garbage collection before loading dep-graph.

michaelwoerister
authored and
cuviper
committed
rustbuild: Restore Config.libdir_relative
cuviper
committed
rustbuild: make libdir_relative a method
cuviper
committed