Adding ability to get file stripe size when on lustre

When compiled with ``lustre`` in the ``CHPL_AUX_FILESYS`` environment variable, this will make it so that when the user calls ``file.getchunk`` from ``IO.chpl`` we will return the stripe size for that file as opposed to the optimal transfer block size.

Note 1: Since the ``CHPL_AUXIO_INCLUDE`` and ``CHPL_AUXIO_LIBS`` might contain extra linkages not directly related to lustre, we need to throw the ``-Qunused-arguments`` flag when compiling with clang.
 
Note 2: When compiled with the Cray or PGI compiler with ``CHPL_AUX_FILESYS = lustre``, warnings are generated from ``posix-types.h`` in ``libcfs`` and ``lustre_user.h`` in ``lustre``, I have been unable to figure out a way to get around these problems. When the plugin is not set, we compile with no warnings (i.e., this is only visible when compiling with the plugin turned on).

@mppf said he would review
It's not totally clear to me why we need sys_lustre_get_stripe_size to be its own exported function (vs e.g. being static in qio.c) but I'm OK with it this way.