need to define LAMMPS_LIB_MPI

doc/src/Library_execute.rst

@@ -32,6 +32,7 @@ Below is a short example using some of these functions.
 
 .. code-block:: C
 
+   #define LAMMPS_LIB_MPI
    #include "library.h"
    #include <mpi.h>
    #include <stdio.h>

examples/COUPLE/simple/simple.c

@@ -23,6 +23,7 @@
 #include <stdlib.h>
 #include <string.h>
 #include <mpi.h>
+#define LAMMPS_LIB_MPI
 #include "library.h"        /* this is a LAMMPS include file */
 
 int main(int narg, char **arg)