** SOLVED ** a friend helped me stumble across the solution. Two things to remember when installing on MacOS High Sierra:
1.) You need to run the install of cpp-driver (which isn't a step in the DataStax instructions referenced in the question) and then
2.) You have to fully qualify the cmake .. command to point to the OpenSSL install.
Here are the amended instructions that worked for me:
# Datastax C++ driver dependencies
brew install libuv cmake
brew install openssl
brew link --force openssl
# Install git if you dont have it
brew install git
# Retrieve the cpp
git clone https://github.com/datastax/cpp-driver.git --depth=1
mkdir cpp-driver/build
cd cpp-driver/build
# Build with qualified path to OpenSSL location
cmake -DOPENSSL_ROOT_DIR=/usr/local/opt/openssl/ -DOPENSSL_LIBRARIES=/usr/local/opt/openssl/lib ..
make
make install
# Install pecl driver
pecl install cassandra
Once that's complete you should be good to go.