Add USE_MPI compilation flag when MPI is enabled

build/configure.ac

@@ -274,6 +274,7 @@ AC_ARG_ENABLE(
   [
     AC_MSG_NOTICE([Handling implicit option.])
     AX_PROG_CXX_MPI()
+    AC_SUBST([MPIFLAGS], [-DUSE_MPI])
   ]
 )
 AS_IF(
@@ -378,7 +379,7 @@ AC_ARG_WITH(
 )
 
 FFLAGS="-std=legacy -O3"
-CXXFLAGS="-O3 -ggdb -Wno-format-contains-nul -I$HDF5_INCLUDE $OMPFLAGS $LAPACKFLAGS $MAGMAFLAGS"
+CXXFLAGS="-O3 -ggdb -Wno-format-contains-nul -I$HDF5_INCLUDE $MPIFLAGS $OMPFLAGS $LAPACKFLAGS $MAGMAFLAGS"
 SUBDIRS="cluster libnptm sphere testing trapping"
 
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