Link options from MPI compiler wrapper collapsed incorrectly (CMake 3.13+)

CMake: 3.14.3
BLT: https://github.com/LLNL/blt/commit/938345715810c6457d3e57a2c20d4ee0fa253632

When using e.g. Intel MPI, link flags obtained from mpicc may look like the following:

-Xlinker -rpath -Xlinker /path/to/A -Xlinker -rpath -Xlinker /path/to/B -Xlinker -rpath -Xlinker /path/to/C ...

After the link options are set on the target in blt_add_target_link_flags via LINK_OPTIONS, the resulting command line looks like:

-Xlinker -rpath /path/to/A /path/to/B /path/to/C ...

which is incorrect. This is because CMake removes duplicates, as per documentation.

Unless there is a way to pass the link flags verbatim (as opposed to splitting and adding one by one), perhaps the correct treatment would be:

1. Collect and concatenate all consecutive options that have a preceding -Xlinker into a comma-separated string
2. Add the resulting string with a LINKER: prefix.

So we end up adding

LINKER:-rpath,/path/to/A,-rpath,/path/to/B,-rpath,/path/to/C

as a single link option, which CMake translates back into the correct string (using -Xlinker or -Wl syntax, as appropriate).

Start at blt_add_target_link_flags and reproduce the issue with the Intel MPI-style flags shown in the report under CMake 3.14.3. Check how LINK_OPTIONS handles repeated -Xlinker arguments, then verify that the generated command preserves each rpath option and path rather than collapsing them.