As per the thread on ticket #4399, OSHMEM does not support non-Linux platforms. So provide a check for Linux and error out if --enable-oshmem is given on a non-supported platform. If no OSHMEM option is given (enable or disable), then don't attempt to build OSHMEM unless we are on a Linux platform. Default to building if we are on Linux for now, pending the outcome of the Debian situation.

cmr=v1.7.5:reviewer=jsquyres:subject=disable OSHMEM on non-Linux platforms
Update README regarding OSHMEM configure options and default behavior.

cmr=v1.7.5:ticket=#4427
Further README tweaks about OSHMEM.

cmr=v1.7.5:ticket=#4427