Enable usingSubsystems if subsystem-dependent setting is used.

If either `defaultSubsystem` or `sitewideLayoutSubsystem` are set
the default value for `usingSubsystems` is changed to true.

Fixes issue #148
Revert "Enable usingSubsystems if subsystem-dependent setting is used."

This reverts commit 48519e0b85b88ceb9f3d632c5ab3722b979aa6e4.
Enable usingSubsystems if subsystem-dependent setting is used.

If either `defaultSubsystem` or `sitewideLayoutSubsystem` are set
the default value for `usingSubsystems` is changed to true.

Fixes issue #148
Merge pull request #175 from boughtonp/develop

Enable usingSubsystems if subsystem-dependent setting is used.