BUGFIX: Reverted use of shlex.

Shlex is only intended to work for Unix-like shells, and using it on
Windows causes problems. We now resort to simply always using the shell
on Unix-like platforms (as our command input is always a string, and not
an array). On Windows, the string input is always okay, even when not
using the shell.

This is a follow-up to a bug report by M. Kammerer on failing Windows
installs.