[build2] LTO and parallelization with build2

[Boris Kolpackov]

Matthew Krupcale <mkrupcale at matthewkrupcale.com> writes:

> Yes, I've pushed changes to the same branch rebased on top of your
> scheduler fix, and things are working much better.

Thanks, I've reviewed the changes and pushed my tweaks:

https://git.build2.org/cgit/build2/log/?h=lto-parallelization

It's mostly cosmetic though there were a few issues (especially
in the Clang part). Could you review them and if happy, I will
merge everything to master.

Note also that I moved the option translation after the low-
verbosity diagnostics. The thinking being that in the -v
output it is more useful to see -flto=auto rather than some
fixed number (e.g., in case the user wants to re-run the
command manually).