"mount/fuse.rs" did not exist on "94c36fa34d7f7003b642f6955a1d8e373bbc84c7"
Somewhat simplify config.mk
- nproc exists on FreeBSD too
- REDOX_MAKE is generally make, so use that as default with per-OS override if necessary.
- HOST_TARGET can be fetched directly from rustc.
Builds on top of !1406 (merged)