cargo 1.19, and later cargo 1.20/rustc 1.19 gained support for the make
jobserver. But that doesn't work unless make passes the jobserver file
descriptors down to subprocesses, which it only does when there is a +
prefixing commands.
--HG--
extra : rebase_source : a00ffb7edb358243ac7b9469941ec6fb5ad84885