If using 'ninja', tell the user to run 'ninja' not 'make' (#4568)

If using 'ninja', tell the user to run 'ninja' not 'make' (#4568)

## Issue

If you call CVC4's `configure` script with `--ninja`, then you get a misleading status message:

```
[avj@tempvm CVC4]$ ./configure.sh --ninja --python3 | tail -n 5
Now change to 'build' and type make, followed by make check or make install.

-- Configuring done
-- Generating done
-- Build files have been written to: /home/avj/working/CVC4/build
```

## Solution

This commit simply fixes the status message to tell the user to run the correct command based on the specified generator:

```
[avj@tempvm CVC4]$ ./configure.sh --ninja --python3 | tail -n 5
Now change to 'build' and type 'ninja', followed by 'ninja check' or 'ninja install'.

-- Configuring done
-- Generating done
-- Build files have been written to: /home/avj/working/CVC4/build
```

Signed-off-by: Andrew V. Jones <andrew.jones@vector.com>