(backported from v2.5)

no actual change

[makedoc]

See #435

[makedoc]

(cherry picked from commit 781e5677)

[makedoc]

Comm::Allgather and Allgatherv now have meaningful defaults when
MPI is not compiled or initialized. This is something that we
discussed in #390

cc: @carlocamilloni

this should make it easier to avoid leaks in the future
(see 04cae903)

There are a few statements resulting in very long outputs on travis-ci
due to warning. I removed them from the code base. In particular:
- a pragma statement
- a non standard use of the C preprocessor