Adjust OpenSSL path when building omnscidb on MacOSX

[pearu]

In cmake output:

Found OpenSSL: /usr/local/opt/openssl/lib/libcrypto.dylib (found version "1.0.2r")

Otherwise, omniscidb builds fine.

To adjust OPENSSL_ROOT for correct $PREFIX, run (when uname is Darwin)

sed -i \"\" -e 's!/usr/local/opt/openssl!\'$PREFIX'!g' CMakeLists.txt

(otherwise, use sed -i 's!...)