Comma symbol between clauses can't be handled

It failed to parse the following code due to `,` symbol between clauses, which is valid syntax.
```c
#pragma omp parallel for private(j,k), collapse(2), lastprivate (jlast, klast)
```

The original failed test cases are from REX compiler.
https://github.com/passlab/rexompiler/blob/master/tests/nonsmoke/functional/CompileTests/OpenMP_tests/collapse.c