Don't pass empty string `[]` as "action-if-not-given" to `AC_ARG_ENABLE()`
macro, as this would otherwise lead to an empty else-block in the resulting
bash `configure` script. This bug was introduced via cb0a503a.

This issue was pointed out by @christiaanb

Reviewed By: christiaanb

Differential Revision: https://phabricator.haskell.org/D545