Do not require an MPI compiler

The AX_PROG_CXX_MPI requires that certain functions are in the MPI
library, but multimpi does define them with different names, to avoid
naming conflicts.

autoconf_build_defines.hxx.in

@@ -48,12 +48,12 @@
 /* SUNDIALS support */
 #undef BOUT_HAS_SUNDIALS
 
-/* Type of the metric fields */
-#undef BOUT_METRIC_TYPE
-
 /* Use libuuid for UUID generation */
 #undef BOUT_HAS_UUID_SYSTEM_GENERATOR
 
+/* Type of the metric fields */
+#undef BOUT_METRIC_TYPE
+
 /* OpenMP schedule */
 #undef BOUT_OPENMP_SCHEDULE
 

configure.ac

@@ -139,9 +139,9 @@ AC_LANG(C++)
 : ${CXXFLAGS="-g"}
 
 # Search for MPI compiler; fail if not found
-AX_PROG_CXX_MPI([], [], [
-  AC_MSG_ERROR([*** An MPI compiler is required. You might need to set MPICXX correctly.])
-])
+dnl AX_PROG_CXX_MPI([], [], [
+dnl   AC_MSG_ERROR([*** An MPI compiler is required. You might need to set MPICXX correctly.])
+dnl ])
 
 # Utility programs
 AC_PROG_MKDIR_P