Add  "-I$srcdir/opal/libltdl/" so conftest will allways set lt_dladvise.

This PR address the conftest compilation problem where gcc is unable to find subsequent includes in ltdl.h:
```c
#include < libltdl/lt_system.h >
#include < libltdl/lt_error.h >
```

@edgargabriel @jsquyres could you verify this?