Implement uname for Redox

See merge request redox-os/relibc!178

Implement alloca.h

See merge request redox-os/relibc!180

Make relibc buildable on macOS (not for macOS)

See merge request redox-os/relibc!179

Don't rely on integer wrapping in ctype and fix strcasecmp on
non-alphabetic characters

The older bash version of macOS does not understand this escape
and actually creates directories with "\" in their name. After this
change it actually works with both versions.

We cannot use ?= because CC is set by default to "cc". Therefore
CC was never set. In addition we need to export the variable
in order to have the rust cc crate pick it up. Otherwise it is
only used by openlibm where it is explicitly passed.