Redefining intrinsic leads to error: has an explicit interface before

Following code fails with the current `F_Front`

```fortran
module mod1

implicit none

private :: gamma
real, private :: cons1, ds, cs

contains

subroutine sub1()
  cons1=gamma(1.+ds)*cs
end subroutine

real function gamma(X)
  real :: X
end function

end module
```

```bash
"test.f90:mod1", line 12: gamma has an explicit interface before
```