Add thread-safe setup support

[flavioheleno]

This PR adds the thread-safe build type to PHP setups.

The structure behind debug builds was slightly changed to better reflect the multiple build type concept (release, debug and thread-safe as defined in unix.sh).

Closes #649.

[shivammathur]

@flavioheleno thanks for working on this.

I added zts debug builds to php-builder today. So I have reverted the debug variable change and introduced a ts variable as both can be set to true.

Would you be working on adding the builds for older versions as well?

[flavioheleno]

@shivammathur I'm all in for adding support to the older versions as well, where are they built?

[shivammathur]

Currently, the project uses builds from ppa:ondrej/php, but it does not have zts builds, so they would have to be added to the php-builder.