PHP: Build with pthreads support

[adamziel]

Pthreads are required to run the pcntl extensions and functions like pcntl_fork and pcntl_exec. Full PHPUnit support hinges on this, and AFAIR there's a few wp-cli commands also relying on threads.

Turns out supporting pthreads offers no clear benefit as WordPress-related PHP libraries typically don't require it. What wp-cli and PHPunit both need is proc_open.

The implementation is explored in #346

Related:

• https://wiki.php.net/rfc/native-tls
• WordPress/playground-tools#25
• http://blog.jpauli.tech/2017-01-12-threads-and-php-html/
• https://stackoverflow.com/questions/49539849/php-internals-how-does-tsrmls-fetch-work
• https://phpinternals.net/categories/extensions#zend-globals

cc @dmsnell @danielbachhuber @sejas