parallel requires a build of PHP with ZTS (Zend Thread Safety) enabled ( --enable-maintainer-zts or --enable-zts on Windows )
Zend Thread Safety cannot be enabled post build; it is a build time configuration option.
parallel should build anywhere there is a working Posix Threads header (pthread.h) and ZTS build of PHP, including Windows (using the pthread-w32 project from redhat).
Windows users can download prebuilt release binaries from the » PECL website.
Windows users need to take the additional step of adding pthreadVC2.dll (distributed with Windows releases) to their PATH.