test: supply empty mtest_mpix.h

When testsuite autogen is run outside mpich, supply an empty
mtest_mpix.h so the testing can proceed. The header files is for mpich
specific extension and will fail with non-mpich implementation anyway.
The empty stub will allow strict testing to proceed.

The regular mpich testing is not affected since mpich's top level
autogen will always supply this file.

test/mpi/autogen.sh

@@ -25,6 +25,18 @@ check_copy version.m4     ../../maint/version.m4
 check_copy confdb         ../../confdb
 check_copy dtpools/confdb ../../confdb
 
+# mtest_mpix.h is generated by mpich autogen, specifically, gen_binding_c.py.
+# It provides macros that replace upcoming MPI functions with MPIX prefix.
+# The tests using these new functions should not be included with
+# --enable-strictmpi, an option should be given when testing general MPI
+# implementations.
+#
+# Here we supply an empty stub when this is run outside mpich.
+#
+if test ! -e include/mtest_mpix.h ; then
+    touch include/mtest_mpix.h
+fi
+
 echo "Running autoreconf in dtpools"
 (cd dtpools && autoreconf -ivf)