Fix MPIEXEC macro definition with previous versions of CMake
#cmakedefine MERCURY_HAS_PARALLEL_TESTING
#ifdef MERCURY_HAS_PARALLEL_TESTING
#cmakedefine MPIEXEC_EXECUTABLE "@MPIEXEC_EXECUTABLE@"
+#cmakedefine MPIEXEC "@MPIEXEC@" /* For compatibility */
#ifndef MPIEXEC_EXECUTABLE
-# cmakedefine MPIEXEC_EXECUTABLE "@MPIEXEC@" /* For compatibility */
+# define MPIEXEC_EXECUTABLE MPIEXEC
#endif
#cmakedefine MPIEXEC_NUMPROC_FLAG "@MPIEXEC_NUMPROC_FLAG@"
#cmakedefine MPIEXEC_PREFLAGS "@MPIEXEC_PREFLAGS@"
