Enable building openssl from source automatically or using command-line flag

[sureshvv]

Have had to do this repeatedly on different systems.

[native-api]

This is not currently supported. If you need to build additional packages from source or otherwise create a custom setup, you are supposed to do that as a separate step before calling pyenv install and provide additional compiler flags to Pyenv if necessary.

The functionality to build OpenSSL and Readline from source was added to only cover the specific case of MacOS without 3rd-party versions of those libraries -- because in that case, those dependencies are present (thus system requirements are technically met) but cannot be used due to Apple's customizations.