Closes https://github.com/davidcole1340/ext-php-rs/issues/208
Closes https://github.com/davidcole1340/ext-php-rs/issues/209
## Summary of the changes
### Build scripts
* the `unix_build.rs` script now honors the `PHP_CONFIG` environment variable, like `cargo php install`
* use `cargo:rerun-if-env-changed` for the `PHP`, `PHP_CONFIG` and `PATH` environment variables, to avoid needless recompilation 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 set up a PHP environment.
* Preliminary Windows support
* Start work on cross-platform build script
* Fix compilation on macOS
* Updated README, tidied up build script
* Check linker version before starting compilation
It doesn't seem like it's possible to change the linker from within the
build script, however, we can retrieve the linker in use and give the
user a suggestion if the linker will not work.
* Switch to using Github repository for bindgen
* Split Windows and Unix implementations into two files
* Fix building on Windows
* Remove `reqwest` and `zip` as dependencies on Unix
* Fix guide tests on Windows
* Started work on Windows CI
* runs -> run
* Use preinstalled LLVM on Windows
* Debugging for Windows CI
* Switch to upstream `rust-bindgen` master branch
* Switch to `rust-lld` for Windows linking
* Don't compile `cargo-php` on Windows
* Switch to using skeptic for tests
* cargo-php: Disable stub generation, fix ext install/remove
The plan is to replace the stub generation by generating them with PHP
code. This is cross-platform and means we don't need to worry about ABI.
We also don't need to embed information into the library.
* cargo-php: Fix on unix OS
* Fix clippy lint
* Updated README
* Re-add CI for Unix + PHP 8.0
* Fix building on thread-safe PHP
* Tidy up build scripts
* Use dynamic lookup on Linux, test with TS Windows
* Define `ZTS` when compiling PHP ZTS
* Combine Windows and Unix CI, fix linking for Win32TS
* Fix exclusions in build CI
* rust-toolchain -> rust
* Set LLVM version
* Only build docs.rs on Ubuntu PHP 8.1
* Fix build on Linux thread-safe
* Update guide example
