deps: allow a --generator switch to update_deps.py

This allows you to use any CMake generator available
on your system to configure the dependent repositories,
allowing you to match the same generator being used by
the project build.

update_deps.py:
- Take a parameter to specify a CMake generator, and pass it to CMake
  if present
- Allow the user to reduce the count of parallel make jobs that can
  happen at one time
- remove an unnecessary semicolon