Incorrect prototype when using constexpr

Using 1.6.10 IDE compiled a few days ago.

This code requires a manual prototype, otherwise the IDE combines the `constexpr` and `int` causing an undeclared type error.

``` C++
constexpr int test();
constexpr int test() { return 5; }

void setup() {
  Serial.begin(9600);
  Serial.println(test());
}

void loop() {
}
```
