Emit a warning when referencing console applications exe's without a main

Create a solution with two Console Application projects:

ConsoleApplication1\Program.fs:

``` f#
module SharedValue
let hello = "Hello" |> id
```

ConsoleApplication2\Program.fs (references ConsoleApplication1)

``` f#
match SharedValue.hello with
| null -> printfn "SharedValue.hello is null!"
| _ -> printfn "all good"
```

Expected output: `all good`
Actual output: `SharedValue.hello is null!`

needless to say, getting a null value from an F# project is quite surprising.

Changing the type of ConsoleApplication1 to a Library fixes this.