One test fails after runtest graph_parallel submodule on x86 paltform.

Environment:
VS 2019 + Windows Server 2016

Issue description:
One test fails after runtest graph_parallel submodule on x86 paltform. Could you please take a look?

Reproduce steps:
git clone ​https://github.com/boostorg/boost.git F:\gitP\boostorg\boost
git -C "F:\gitP\boostorg\boost" submodule update --init --recursive
.\b2 headers variant=release --build-dir=..\out\x86rel address-model=32
.\b2 -j16 variant=release --build-dir=..\out\x86rel libs\graph_parallel\test
[test_38.log](https://github.com/boostorg/graph_parallel/files/7097654/test_38.log)

ErrorMessage:
compile-c-c++ ..\out\x86rel\boost\bin.v2\libs\graph_parallel\example\msvc-14.2\release\threading-multi\breadth_first_search.obj
breadth_first_search.cpp
.\boost/mpi/config.hpp(22): fatal error C1083: Cannot open include file: 'mpi.h': No such file or directory (d:\a01\_work\4\s\src\vctools\Compiler\CxxFE\sl\p1\c\p0prepro.c:1964)

...failed compile-c-c++ ..\out\x86rel\boost\bin.v2\libs\graph_parallel\example\msvc-14.2\release\threading-multi\breadth_first_search.obj...

compile-c-c++ ..\out\x86rel\boost\bin.v2\libs\graph_parallel\example\msvc-14.2\release\threading-multi\dijkstra_shortest_paths.obj
dijkstra_shortest_paths.cpp
.\boost/mpi/config.hpp(22): fatal error C1083: Cannot open include file: 'mpi.h': No such file or directory (d:\a01\_work\4\s\src\vctools\Compiler\CxxFE\sl\p1\c\p0prepro.c:1964)

...failed compile-c-c++ ..\out\x86rel\boost\bin.v2\libs\graph_parallel\example\msvc-14.2\release\threading-multi\dijkstra_shortest_paths.obj...

Inizia con il comando di test graph_parallel e il file test_38.log allegato, quindi esamina boost/mpi/config.hpp e le build non riuscite di breadth_first_search.cpp e dijkstra_shortest_paths.cpp. Riproduci la build x86 release su VS 2019 e determina perché mpi.h non è disponibile; il lavoro è completato quando il test graph_parallel termina senza gli errori di compilazione segnalati.

Scritto dal modello di indicizzazione a partire dal testo della issue.