Avoid uninitialized CMake GETOPT variable warnings

Avoid uninitialized CMake GETOPT variable warnings

Only access GETOPT_INCLUDE_DIRS if GETOPT is found.

Fix a few warnings with --warn-uninitialized like:

```
CMake Warning (dev) at cmd/gvmap/CMakeLists.txt:20
(target_include_directories):
  uninitialized variable 'GETOPT_INCLUDE_DIRS'
```

Towards #2291