can not compile plugins with 9.6

[tjhei]

Starting from a clean build directory with deal.II 9.6 installed through candi, I can not compile plugins (as part of the tests):

$ ctest -R plugin
1/6 Test  #813: plugin ...........................***Failed    4.62 sec
In file included from /ssd/libs-candi-9.6/deal.II-dealii-9.6/include/deal.II/base/timer.h:18,
                 from /ssd/aspect-git5/include/aspect/simulator.h:25,
                 from /ssd/aspect-git5/tests/plugin.cc:22:
/ssd/libs-candi-9.6/deal.II-dealii-9.6/include/deal.II/base/config.h:606:12: fatal error: mpi.h: No such file or directory

I found #6176 which seems to be related.