Add support for debug builds

[kocsismate]

Describe the feature I'm a PHP extension maintainer and core contributor, so it would make my work much easier if setup-php would support using debug builds (./configure --enable-debug).

Underlying issue Debug builds have additional error checks and validations enabled, and these are essential for extension development.

Are you willing to submit a PR? I cannot submit a PR because I prefer working on PHP core in my free time.