Adds support for Debian 10 ('buster')

This should be a slightly better fix than 088528a5

This reverts commit 088528a5.

0.7 seconds makes it way too sensitive to setup time

OpenMPI defaults have changed in buster, so we need to enable oversubscribing explicitly.

Removes -Werror
See https://github.com/open-mpi/ompi/blob/master/ompi/mpi/cxx/intercepts.cc#L133-L241

This fixes the build on trusty

Having some trouble running with the foss toolchain, else I'd include them instead.

Downloading and placing all required tarballs in the correct places
is likely to be tedious.

This is needed because OpenMPI on trusty fails with this error:

/usr/lib/openmpi/include/mpi_portable_platform.h:374:34: error: invalid suffix on literal; C++11 requires a space between literal and string macro [-Werror=literal-suffix]
              _STRINGIFY(__GNUC__)"."_STRINGIFY(__GNUC_MINOR__)"."_STRINGIFY(__GNUC_PATCHLEVEL__)

They're probably not fully complete