Including `mpiext_*_mpifh.h` in the source file of the `mpi_f08_ext`
module is not always appropriate. For example, if you want to define
a new datatype in an MPI extension, the `include 'mpif-ext.h'` binding
defines the datatype as `integer` but the `use mpi_f08_ext` binding
defines it as `type(mpi_datatype)`. They conflict.
This commit allows each MPI extension to declare whether it wants to
include its `mpiext_*_mpifh.h` in `mpi_f08` and `mpi_f08_ext`
respectively. The default (no declaration) is 'want'.
See `ompi/mpiext/example/configure.m4` for an example.
Signed-off-by: KAWASHIMA Takahiro <t-kawashima@jp.fujitsu.com>
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