[PHP] Upgrade GD to 2.3.3

[adamziel]

Motivation for the change, related issues

Upgrades GD version to 2.3.3. The EWWW Image Optimizer relies on GD library features shipped with a more recent GD version than the one bundled with PHP. See nosilver4u/ewww-image-optimizer#377

Implementation details

PHP comes with a bundled GD version 2.1.0. Upgrading to 2.3.3 involves:

• Downloading and building 2.3.3 as a separate library (living in packages/php-wasm/compile/libgd)
• Using it while compiling PHP 8.1+ via --with-external-gd
• Falling back to the bundled libgd==2.1.0 for PHP <= 8.0. These older PHP releases don't seem to be compatible with newer GD and fail to build due to multiple missing symbols, e.g. /root/php-src/ext/gd/gd.c:3536:8: error: use of undeclared identifier 'GD_FLIP_VERTICAL'.

Testing Instructions (or ideally a Blueprint)

CI – we have tests covering gd across all PHP versions.

[adamziel]

There's some superficial differences in whitespaces, I'll adjust them in a follow-up.