Define WIN32_LEAN_AND_MEAN before including Windows Platform SDK
headers to preempt naming conflicts and to make the build faster. Avoid
including those headers in BoringSSL headers. Document that Platform
SDK 8.1 or later is required on Windows.
Change-Id: I907ada21dc722527ea37e839c71c5157455a7003
Reviewed-on: https://boringssl-review.googlesource.com/3100
Reviewed-by: Adam Langley <agl@google.com>
Initial fork from f2d678e6e89b6508147086610e985d4e8416e867 (1.0.2 beta).
(This change contains substantial changes from the original and
effectively starts a new history.)
