FindOpenACC: Provide a Fortran snippet that compiles with gfortran

The original Fortran snippet would fail to compile with the error
message of:
```
  return 0;
         1
Error: Alternate RETURN statement at (1) is only allowed within a SUBROUTINE
```

We solve this by removing the early termination logic.

Modules/FindOpenACC.cmake

@@ -60,9 +60,7 @@ int main(){
 set(OpenACC_Fortran_TEST_SOURCE
 "
 program test
-#ifdef _OPENACC
-  return 0;
-#else
+#ifndef _OPENACC
   breaks_on_purpose
 #endif
 endprogram test