Cannot include `mpi-ext.h` without `mpi.h`

[eschnett]

In a project of mine I am using the cmake statement check_include_file(mpi-ext.h HAVE_MPI_EXT_H) to see whether MPI provides the file mpi-ext.h. Surprisingly this check fails for OpenMPI 5.0.0. The error message is

FAILED: CMakeFiles/cmTC_aa701.dir/CheckIncludeFile.c.o 
/Users/eschnett/openmpi-5.0.0/bin/mpicc   -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX14.0.sdk -std=gnu99 -o CMakeFiles/cmTC_aa701.dir/CheckIncludeFile.c.o -c /Users/eschnett/src/c/MPItrampoline6/mpiwrapper/build-openmpi/CMakeFiles/CMakeTmp/CheckIncludeFile.c
In file included from /Users/eschnett/openmpi-5.0.0/include/mpi-ext.h:16,
                 from /Users/eschnett/src/c/MPItrampoline6/mpiwrapper/build-openmpi/CMakeFiles/CMakeTmp/CheckIncludeFile.c:1:
/Users/eschnett/openmpi-5.0.0/include/openmpi/mpiext/mpiext_affinity_c.h:24:14: error: expected ';' before 'int'
   24 | OMPI_DECLSPEC int OMPI_Affinity_str(ompi_affinity_fmt_t fmt_type,
      |              ^~~~
      |              ;

The reason seems to be that the file mpi-ext.h includes openmpi/mpiext/mpiext_affinity_c.h, which uses the symbol OMPI_DECLSPEC without defining it first. This symbol is defined in mpi.h, but mpi-ext.h does not include mpi.h.

[bosilca]

First, this is not a self-contained file, it cannot function without a properly installed OMPI. Second, this did not change with the 5.0 release, it was exactly the same before with the 4.1 stable.

[devreal]

I would argue that the order in which applications include mpi.h and mpi-ext.h should not matter. So whatever mpi-ext.h uses must be included.

[jsquyres]

@bosilca is correct: this hasn't changed since v4.1.x; it would be weird if it doesn't fail there.

That being said, I don't have any objection to making <mpi-ext.h> include <mpi.h> so that it is self-contained. See #12122.