Improve the documentation in BUILDING

The initial instructions given don't work on Windows for a variety of
reasons. Document the prerequisite tools and the limitations on
Windows.

BUG=crbug.com/451610

Change-Id: Ib5eaf00ed9b91c02b4d0e9987f8f3b4eb73266d3
Reviewed-on: https://boringssl-review.googlesource.com/3010
Reviewed-by: Adam Langley <agl@google.com>