build regression w/ current GASNet-EX and smp-conduit/mpi-spawner

[Paul Hargrove]

Since merge od GASNet PR#367, GASNet's smp-conduit has supported spawning via ssh, mpi and pmi.

This has the side-effect that when MPI is detected at configure time, one must link smp-conduit executables with mpicc/mpicxx unless additional action is taken. Note that this makes smp-conduit no different from ibv and ofi, and therefore (IMO) this is very a minor issue to be addressed in INSTALL.md

Example:

$ [path-to]/configure --with-gasnet=https://bitbucket.org/berkeleylab/gasnet/get/develop.tar.gz --with-default-network-smp
[...output elided...]
$ make
[...most of output elided...]
ERROR: MPI C++ link test failed (required by: smp mpi)
ERROR: In most cases configuring UPC++ using `--with-cxx=mpicxx` (or similar)
ERROR: will resolve this issue.  However, 'Configuration: Linux' in INSTALL.md
ERROR: includes additional options and more information.
[...about ten more lines of output elided...]

For any user encountering this issue, see the Configuration: Linux section in INSTALL.md for text which begins "By default ibv-conduit (InfiniBand support) will use MPI for job spawning..." and mentally replace ibv with smp.