v4.0.x: Allow user to overwrite OMPI_ENABLE_MPI1_COMPAT
Follow-up to #6120.
As mentioned in [1], it may be desirable to nevertheless get the hidden
MPI 1 prototypes, for users who know what they are doing, i.e., the tools
guys. @ggouaillardet mentioned in [2], that `-DOMPI_OMIT_MPI1_COMPAT_DECLS=0`
should work, but it does not, as than we only get redefinition warnings.
See [3].
This topic does not relate to master, as we can remove the actual symbols
there, but here in v4.0.x land, the symbols are always there.
[1] https://github.com/open-mpi/ompi/pull/6120#issuecomment-443104700
[2] https://github.com/open-mpi/ompi/pull/6120#issuecomment-443117892
[3] https://github.com/open-mpi/ompi/pull/6120#issuecomment-468962596
Signed-off-by: Bert Wesarg <bert.wesarg@tu-dresden.de>
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