Fix comment labelling relibc as newlib

See merge request !58

This reverts commit f05f2e4f.
Turns out Jeremy already fixed it!

Since bash variables are global my last changes accidentally overwrote the action. Fixed by assigning the original value to a new variable

Fix setup.sh

See merge request redox-os/libc!57

1. Actually compile the redox version of relibc
2. Use newlib by default since relibc isn't usable yet
3. Check for binaries like autoconf without version in the name - only
to check for the version later.

update newlib PKGBUILD to use rust nightly 2018-04-27 as setup.sh does

Make more NixOS friendly :)

NixOS users can now run 'nix-shell' to automatically fetch all dependencies. They still need to patch setup.sh to find autoconf under the name "autoconf" and not "autoconf2.64", but it's better than nothing!

Remove "-j $(nproc)" from make command

ArchLinux users may set MAKEFLAGS in /etc/makepkg.conf
(https://wiki.archlinux.org/index.php/Makepkg#Parallel_compilation).

Fix redox-os/newlib#78

Building newlib failed with latest nightly (1.27.0 as of this commit),
and using previous nightly (1.26.0, or 2018-03-26) works.