This gets cURL building against both BoringSSL as it is and BoringSSL
with OPENSSL_VERSION_NUMBER set to 1.1.0.

BUG=91

Change-Id: I5be73b84df701fe76f3055b1239ae4704a931082
Reviewed-on: https://boringssl-review.googlesource.com/10180
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>
Commit-Queue: David Benjamin <davidben@google.com>
Commit-Queue: Adam Langley <agl@google.com>
Reviewed-by: Adam Langley <agl@google.com>