Auto merge of #3332 - alexcrichton:build-openssl, r=alexcrichton

Compile OpenSSL from source on OSX

I'm seeing a bunch of weird illegal instructions on OSX nightlies for
Cargo. My guess is that they're all related to OpenSSL linking. Right
now we're linking from Homebrew but I have a sneaking suspicion that it
compiles with `-march=native` rather than what we'd like as a portable
binary. To work around this compile OpenSSL ourselves and link it that
way.

Note that I believe this won't bring in the certificate trust store of
OpenSSL on OSX (or at least not the right one from the keychain). We
shouldn't need that, however, as OpenSSL is just used as the
cryptographic primitives in libssh2 and Cargo itself. So in that sense
we shouldn't need it for actually SSL at all.