STK without MPI: fatal error: mpi.h: No such file or directory

Created by: nschloe

When compiled with TPL_ENABLE_MPI=OFF, stk fails to compile with

[...]
[ 70%] Building CXX object packages/stk/stk_util/stk_util/parallel/CMakeFiles/trilinos_stk_util_parallel.dir/ParallelIndexGapFinder.cpp.o
In file included from /home/nschloe/software/trilinos/privateTrilinos/packages/stk/stk_util/stk_util/parallel/ParallelIndexGapFinder.cpp:34:0:
/home/nschloe/software/trilinos/privateTrilinos/packages/stk/stk_util/stk_util/parallel/ParallelIndexGapFinder.hpp:41:19: fatal error: mpi.h: No such file or directory
 #include "mpi.h"
[...]