[CI] Add an option to select the generator

Summary:
This let the builds run with another generator instead of ninja.
make is special cased to include the -jN+1 option with N being the
CPU count.

Test Plan:

./build-configurations.py build-make-generator

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D7397