There are two generic OS dependencies that will be the most common, unix and windows.
Other options are linux, freebsd, darwin (OS X), sunos, irix, hpux, aix.
As with all dependency validation, true is returned on success, PEAR_Error on failure
for required dependencies (and the OS dependency is required). If the
options are specified, an array containing the error message will be returned instead.
Dependency contents as parsed from xml
throws no exceptions thrown
This function can not be called statically.