[cmake] When building against MPICH, mpi_c target shows libmpicxx in INTERFACE_LINK_LIBRARIES

[Wentzell]

Description

libmpicxx should not be required to compile the TRIQS/mpi headers, but is listed in the INTERFACE_LINK_LIBRARIES
of the mpi::mpi_c cmake target, only when building against MPICH.

See e.g. PREFIX/lib/cmake/mpi/mpi-targets.cmake file for the py38h82d08ca_3 build of the https://github.com/conda-forge/triqs-feedstock

 67 set_target_properties(mpi::mpi PROPERTIES
 68   INTERFACE_COMPILE_DEFINITIONS "MPICH_SKIP_MPICXX;OMPI_SKIP_MPICXX;_MPICC_H"
 69   INTERFACE_INCLUDE_DIRECTORIES "PREFIX/include"
 70   INTERFACE_LINK_LIBRARIES "PREFIX/lib/libmpicxx.dylib;PREFIX/lib/libmpi.dylib;PREFIX/lib/libpmpi.dylib"
 71   INTERFACE_SYSTEM_INCLUDE_DIRECTORIES "PREFIX/include"
 72 )

Versions

Confirmed for Version 1.0.0