This package uses the (La)TeX extension -shell-escape to
establish whether the document is being processed on a Windows
or on a Unix-like system (Mac OS X, Linux, etc.), or on Cygwin
(Unix environment over a windows system). Booleans provided
are \ifwindows, \iflinux, \ifmacosx, and \ifcygwin. The
package also preserves the output of uname on a Unix-like
system, which may be used to distinguish between various
classes of Unix systems.