Update install-sx.sh

Dear Mr Amir Taaki, I made some changes to the install script to make it multi-OS friendly, and also added some compilation tips for less experienced users. This is a pull request for you to merge these changes to your main branch.

Heres a document explaining how to enable EC for openssl on Fedora/Redhat
https://docs.google.com/document/d/1VgdhCdpRhAuDVE6pnwx6oKyLRasDvOKtCnXKuOLhEB8/edit?usp=sharing

From,
Hugh Pearse,
Ireland.