Bug#943313: liggghts should not use MPI_C_COMPILER or MPI_CXX_COMPILER

[Helmut Grohne]

Source: liggghts
Version: 3.8.0+repack1-4
Tags: patch upstream
User: debian-cross at lists.debian.org
Usertags: ftcbfs

liggghts uses MPI and it uses it via FindMPI.cmake, which presently
detects MPI using compiler wrappers such as mpicc. During cross
compilation these wrappers will not work and they will be unsupported on
Debian. Beyond breaking cross compilation, such wrappers break when you
need two of them (e.g. ccache/distcc/mpicc). For MPI, a good solution is
using pkg-config and doing so is well supported by all major
implementations. I'm working on a patch for FindMPI.cmake to make it
work with pkg-config. However that'll result in MPI_C_COMPILER and
MPI_CXX_COMPILER being unusable. Unfortunately, liggghts uses them.
FindMPI.cmake also provides the relevant compiler and linker flags via
other cmake variables, so you can stop using MPI_*_COMPILER today and
thus enable cross building once FindMPI.cmake gains support for it.
Please consider applying the attached patch. Doing so early helps with
developing the cmake patch. I'm attaching my current draft for
reference.

Helmut
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