macOS/prerequisites: brew boostrap missing.

* As hinted by @misl6 the prerequisites is the place where whole build environment can be bootstrapped.
* On macOS `brew` is used to install packages, prerequisites depends on it, but cannot yet install it on its own.
* This can be trivially fixed by using script provided at HomeBrew website [1]:
```
/bin/bash -c "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/HEAD/install.sh)"
```
* When p4a.prerequisites bootstraps non existing brew on its own, a particular version may be selected (?) with particular packages, and this may result in more coherent build environment + far better automation.
* I am on the trip right now, not really familiar yet with the prerequisites, so this may be my first task in p4a, unless someone makes it faster / better :-)

[1] https://brew.sh/

Commencez au point d’entrée p4a.prerequisites et suivez la manière dont macOS vérifie la présence de brew et l’utilise. Vérifiez le comportement lorsque brew est absent, puis confirmez que l’amorçage des prérequis s’achève avec la configuration de paquets requise et préserve le chemin existant lorsque brew est déjà installé.

Rédigé par le modèle d'indexation à partir du texte de l'issue.