honour PHP_CONFIG & rebuild automatically when env vars change

[ju1ius]

Closes #208.
Closes #209.

Please see the linked issues for a rationale.

Summary of the changes

Build scripts

• the unix_build.rs script now honours the PHP_CONFIG environment variable, like cargo php install
• use cargo:rerun-if-env-changed for the PHP, PHP_CONFIG and PATH environment variables, in order to avoid needless recompilations of the whole dependency tree.

Documentation

While trying to document the aforementioned changes, I realized that there was no chapter about installing and setting up a PHP environment to develop PHP extensions. So I refactored the first chapters of the book into a Getting Started section, including instructions on how to quickly setup a PHP environment.

Do not hesitate to point-out spelling/grammar mistakes as I'm not a native english speaker.

[joehoyle]

I think this might have caused an issue for me. I'm seeing

     Fresh skeptic v0.13.7
   Compiling ext-php-rs v0.10.0
     Running `.../debug/build/ext-php-rs-a3bbbf43415ad934/build-script-build`

On every cargo check / cargo build which takes maybe 3+ seconds, which also makes Rust-Analyzer super slow. Is the intention here that if the env vars don't change, the build should still be cached? I had to switch back to 0.8.3 where I get fully cached builds again

[ptondereau]

@joehoyle hey can you submit a ticket please, I'll take a look! BTW while working on it, I didn't see that regression

[ju1ius]

Is the intention here that if the env vars don't change, the build should still be cached?

Yes, that's how cargo:rerun-if-env-changed is supposed to work, unless something else is interfering...

[ju1ius]

Found a 3 years old issue on the cargo repo that might be related.
Maybe it's worth investigating?