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.

Fix include guards in regex and utime

See merge request !176

The include_guard in both regex and utime was "_TEMPLATE_H", this meant that including both wasn't possible.
Their header guards should be consistent with the header files as well.
Changed _TEMPLATE_H to _REGEX_H and _UTIME_H in /regex/cbindgen.toml and /utime/cbindgen.toml respectively.

Implement libgen.h

See merge request !175

Implemented the following calls according to http://pubs.opengroup.org/onlinepubs/7908799/xsh/libgen.h.html
- char* basename(char*)
- char* dirname(char*)

Added test suit for the implemented calls.

Issue: redox-os/relibc#134

Handle getopt reinitialization

See merge request redox-os/relibc!174

Disable symbol mangling for C++

See merge request redox-os/relibc!173

Fix assert when used as an expression

See merge request !172

Based on what musl does.