diff --git a/build/configure.ac b/build/configure.ac
index 2acdc4710a063abfb4794a367db61717fbe1cc65..9b8c875296bec12dace06d716595b1d930d755af 100644
--- a/build/configure.ac
+++ b/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"
 
 # Send output to Makefiles