opal_check_ident.m4: make the test match what we do in the code base

Per open-mpi/ompi#257, the configure test used to check:

```c
#pragma ident "foo"
```

But out in the code base, we actually do this:

```c
#define IDENT_MSG "foo"
// ...
#pragma ident IDENT_MSG
```

This commit updates the configure test to ```#define``` a string and then use that with the various ident strategies.