Support building with BoringSSL

Support building with BoringSSL

BoringSSL is Google's cleanup of OpenSSL and an attempt to unify
Chromium, Android and internal codebases around a single OpenSSL.

As part of moving Android to BoringSSL, the wpa_supplicant maintainers
in Android requested that I upstream the change. I've worked to reduce
the size of the patch a lot but I'm afraid that it still contains a
number of #ifdefs.

[1] https://www.imperialviolet.org/2014/06/20/boringssl.html

Signed-off-by: Adam Langley <agl@chromium.org>