pthreads requires a build of PHP with ZTS (Zend Thread Safety) enabled (--enable-zts, or on non-Windows systems prior to PHP 8.0.0, --enable-maintainer-zts)
Zend Thread Safety cannot be enabled post build; it is a build time configuration option.
pthreads 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).