SI-7962 Scalac runner does not work within Emacs's terminal

  - Always set the env.emacs system property - scalac only cares
    about whether the system property has a non-empty value, not
    whether it is set or not. Fixes 7962
Merge pull request #3574 from greenrd/SI-7962

SI-7962 Scalac runner does not work within Emacs's terminal