Revert "Re #6902. Remove unnecessary code from linux build."

This appears to have had a bad effect on our fragile MantidPlot
python tests and I'm not inclined to figure out what's wrong
exactly (they still work locally for me).

This reverts commit 424a87b8fd794b5573ac709a527a02b291f9f604.
Merge branch 'feature/6902_built_and_installed_on_osx' into develop