Clean up use of Windows Platform SDK headers.

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-by: Adam Langley <>
14 files changed