`MPIX_C_FLOAT16` is defined as a synonym for `MPIX_SHORT_FLOAT`
if the C compiler supports `_Float16`, which is defined in
ISO/IEC JTC 1/SC 22/WG 14 N1945 (ISO/IEC TS 18661-3:2015).
This name and meaning are same as that of MPICH. This may be
a transitional datatype until the MPI Forum decides a proper
name for the type.
Signed-off-by: KAWASHIMA Takahiro <t-kawashima@jp.fujitsu.com>
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