Add posibility to build as a shared library.
Add posibility to build as a shared library.

israelins85/SmtpClient-for-Qt,cutelyst/simple-mail,LangeSebastian/simple-mail,Spiek/SmtpClient-for-Qt,Spiek/SmtpClient-for-Qt,LangeSebastian/simple-mail,israelins85/SmtpClient-for-Qt,amiraiak/AA,amiraiak/AA,cutelyst/simple-mail