Name clash with multiple python impls

On Gentoo, we usually build Boost with multiple python impls. Since 1.72 we are getting name clashes when building with MPI and multiple python impls:
```
error: Name clash for '

mpi.so'
error:
error: Tried to build the target twice, with property sets having
error: these incompatible properties:
error:
error: - /usr/include/python2.7 2.7 /var/tmp/portage/dev-libs/boost-1.72.0/work/boost_1_72_0-abi_x86_64.amd64/bin.v2/libs/mpi/build/gcc-9.2/gentoorelease/local-visibility-global/pch-off/python-2.7/threading-multi/visibility-hidden /var/tmp/portage/dev-libs/boost-1.72.0/work/boost_1_72_0-abi_x86_64.amd64/bin.v2/libs/python/build/gcc-9.2/gentoorelease/pch-off/python-2.7/threading-multi/visibility-hidden
error: - /usr/include/python3.6m 3.6 /var/tmp/portage/dev-libs/boost-1.72.0/work/boost_1_72_0-abi_x86_64.amd64/bin.v2/libs/mpi/build/gcc-9.2/gentoorelease/local-visibility-global/pch-off/python-3.6/threading-multi/visibility-hidden /var/tmp/portage/dev-libs/boost-1.72.0/work/boost_1_72_0-abi_x86_64.amd64/bin.v2/libs/python/build/gcc-9.2/gentoorelease/pch-off/python-3.6/threading-multi/visibility-hidden
```
this is very likely due to the change in https://github.com/boostorg/mpi/commit/3ecbf83ff33750ceffda199de97283d200357633. @Lastique @pdimov any ideas on how we could fix this? I tried injecting all sorts of tags into the Jamfile, but to no avail, it always crashes with the name clash error above. Our bug report: https://bugs.gentoo.org/703036 building it against a single python impl solves the issue, but isn't a tractable solution for gentoo users.