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/

Start at the p4a.prerequisites entry point and trace how macOS checks for and uses brew. Verify the behavior when brew is absent, then confirm that prerequisite bootstrapping completes with the required package setup and preserves the existing path when brew is already installed.

Written by the indexing model from the issue text.