fix detection of python version 3

configure errors:
../mpich/configure: line 21398: python: command not found
../mpich/configure: line 21398: test: 3: unary operator expected

confdb/aclocal_misc.m4

@@ -3,17 +3,33 @@ dnl
 AC_DEFUN([PAC_CHECK_PYTHON],[
     AC_ARG_VAR([PYTHON], [set to Python 3])
     if test -z "$PYTHON" ; then
-        AC_MSG_CHECKING([Python 3])
         PYTHON=
         python_one_liner="import sys; print(sys.version_info[[0]])"
-        if test 3 = `python -c "$python_one_liner"`; then
-            PYTHON=python
-        elif test 3 = `python3 -c "$python_one_liner"`; then
-            PYTHON=python3
+
+        dnl check command 'python'
+        PYTHON_PATH=
+        AC_PATH_PROG(PYTHON_PATH, python)
+        if test "x$PYTHON_PATH" != x ; then
+            py_version=`$PYTHON_PATH -c "$python_one_liner"`
+            if test "x$py_version" = x3 ; then
+                PYTHON=$PYTHON_PATH
+            fi
+        fi
+        dnl PYTHON is still not set, check command 'python3'
+        if test "x$PYTHON" = x ; then
+            PYTHON3_PATH=
+            AC_PATH_PROG(PYTHON3_PATH, python3)
+            if test "x$PYTHON3_PATH" != x ; then
+                py3_version=`$PYTHON3_PATH -c "$python_one_liner"`
+                if test "x$py3_version" = x3 ; then
+                    PYTHON=$PYTHON3_PATH
+                fi
+            fi
         fi
-        AC_MSG_RESULT($PYTHON)
         if test -z "$PYTHON" ; then
-            AC_MSG_WARN([Python 3 not found! Bindings need to be generated before configure.])
+            AC_MSG_WARN([Python version 3 not found! Bindings need to be generated before configure.])
+        else
+            AC_MSG_NOTICE([Python version 3 is $PYTHON])
         fi
     fi
 ])