sha1: allow selecting SHA-1 library at build time
Add a #define which allows selecting between the sha1collisiondetection and
openssl SHA-1 implementations at build time.
For now setup.py always uses sha1collisiondetection, but we have some other
internal build environments where we will likely default to OpenSSL's SHA-1
Test Plan: Confirmed the code builds and passes tests.
Reviewers: #fbhgext, ryanmce
Reviewed By: #fbhgext, ryanmce
Differential Revision: https://phab.mercurial-scm.org/D285