Add an fpm-zts build

[RulerOf]

Resolves #1317, #249

To echo this statement from @okdewit, I have a Symfony application using the PHP Parallel API that I cannot run correctly under FPM without ZTS.

It's not clear if there's a formal testing procedure here, but I can validate that I'm able to install the parallel extension in the php 7.4 container I've built:

╰─❯ docker run -it --rm php:7.4-fpm-zts bash
root@5c46acc8317c:/var/www/html# pecl install parallel-1.1.4
downloading parallel-1.1.4.tgz ...
Starting to download parallel-1.1.4.tgz (58,783 bytes)
..............done: 58,783 bytes
42 source files, building
<snip>
Build process completed successfully
Installing '/usr/local/lib/php/extensions/no-debug-zts-20190902/parallel.so'
install ok: channel://pecl.php.net/parallel-1.1.4
configuration option "php_ini" is not set to php.ini location
You should add "extension=parallel.so" to php.ini

[RulerOf]

Will reopen from a different branch