chore(build): mpi: set mpicxx, too.

[aws-nslick]

Stacked PRs:

• feat(test): parse as c++ source #568
• ->chore(build): mpi: set mpicxx, too. #567
• chore(build): add AC_PROG_CXX #566
• feat(rdma): constrain C linkage to init #591
• feat(build): add -Wextra to "picky" compiler flags #588
• fix(platform-aws): fill all platform values #595
• fix(tree): use empty brace initializers for zero-initialization #594
• fix(tracing/nvtx): silence -Wmissing-field-initializer warnings #593
• fix(build): check features before mangling CFLAGS #589
• fix(test): fix typing issues #587
• fix(neuron): remove const from ncclNetPlugin_v{4,5} syms #577
• fix(sendrecv): add missing nccl-headers include #576
• fix(rdma): avoid enum/integral comparison #586
• fix(tree): add fallthrough switch markers #585
• fix(rdma): use COMM_ID_MASK as invalid id #574
• fix(tree): avoid sign comparison issues #575
• fix(param): move some parameters to unsigned #571
• feat(param): add uint parameter macro #570
• fix(tuner): fix implicit conversions #573
• fix(tuner): avoid gotos #569
• fix(tree): use decltype instead of typeof for cxx #565
• fix(tree): move declarations to top of function #563

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chore(build): mpi: set mpicxx, too.

Signed-off-by: Nicholas Sielicki [email protected]

The base branch was changed.

[aws-nslick]

Squashed this into #568