Solve problem with generated test sh-scripts

If compiled with MPI, the necessary "mpiexec -n 2" was not written
in the check scripts. This commit is based on a patch provided by
Michael Banck from debian.org

Makefile.am

@@ -306,14 +306,15 @@ endif
 
 
 # test scripts
+MPIEXEC ?= mpiexec -n 2
 if WITH_MPI
-  wrapper="mpiexec -n 2 "
+  wrapper=$(MPIEXEC)
 else
-  wrapper=""
+  wrapper=
 endif
 TESTS = $(check_SCRIPTS)
 %.sh: %
-	echo '$(wrapper)./$^ $$TEST_FLAGS' > $@
+	echo '$(wrapper) ./$^ $$TEST_FLAGS' > $@
 	chmod +x $@
 
 ## this one does not want any arguments

configure.ac

@@ -70,7 +70,7 @@ fi
 
 dnl mpi
 AC_ARG_WITH(mpi, [AS_HELP_STRING([--with-mpi=[[yes|no]]], [compile with MPI. Default: yes])],,[with_mpi=yes])
-AM_CONDITIONAL([WITH_MPI],[test x"with_mpi" = x"yes"])
+AM_CONDITIONAL([WITH_MPI],[test x"$with_mpi" = x"yes"])
 if test x"${with_mpi}" = x"yes"; then
        AC_DEFINE([WITH_MPI], [1], [use MPI])
 fi