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.
Merge pull request #70 from jsquyres/pr/fix-ident-configury

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