Fix a warning in root 'CMakeLists.txt'

This fixes a warning introduced by dd80835:

```
CMake Warning (dev) in CMakeLists.txt:
   A logical block opening on the line

     CMakeLists.txt:152 (if)

   closes on the line

     CMakeLists.txt:154 (endif)

   with mis-matching arguments.
 This warning is for project developers.  Use -Wno-dev to suppress it.
```