macOS/prerequisites: brew boostrap missing.

[cederom]

• 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/