build: fix tools/configure in case only python3 exists

Calling ./configure with python3 being there but no python,
tools/configure will fail. Fix that by defaulting to python and
falling back to python3 or python2.

While at it fix the use of non portable "type -p" by replacing it by
AC_PATH_PROG().

Signed-off-by: Juergen Gross <jgross@suse.com>
Acked-by: Wei Liu <wl@xen.org>
[ wei: run autogen.sh ]
Reviewed-by: Ian Jackson <ian.jackson@eu.citrix.com>

tools/configure

@@ -6926,12 +6926,96 @@ then
 fi;;
 esac
 if test -z "$PYTHON"; then :
-  PYTHON="python"
+  for ac_prog in python python3 python2
+do
+  # Extract the first word of "$ac_prog", so it can be a program name with args.
+set dummy $ac_prog; ac_word=$2
+{ $as_echo "$as_me:${as_lineno-$LINENO}: checking for $ac_word" >&5
+$as_echo_n "checking for $ac_word... " >&6; }
+if ${ac_cv_prog_PYTHON+:} false; then :
+  $as_echo_n "(cached) " >&6
+else
+  if test -n "$PYTHON"; then
+  ac_cv_prog_PYTHON="$PYTHON" # Let the user override the test.
+else
+as_save_IFS=$IFS; IFS=$PATH_SEPARATOR
+for as_dir in $PATH
+do
+  IFS=$as_save_IFS
+  test -z "$as_dir" && as_dir=.
+    for ac_exec_ext in '' $ac_executable_extensions; do
+  if as_fn_executable_p "$as_dir/$ac_word$ac_exec_ext"; then
+    ac_cv_prog_PYTHON="$ac_prog"
+    $as_echo "$as_me:${as_lineno-$LINENO}: found $as_dir/$ac_word$ac_exec_ext" >&5
+    break 2
+  fi
+done
+  done
+IFS=$as_save_IFS
+
+fi
+fi
+PYTHON=$ac_cv_prog_PYTHON
+if test -n "$PYTHON"; then
+  { $as_echo "$as_me:${as_lineno-$LINENO}: result: $PYTHON" >&5
+$as_echo "$PYTHON" >&6; }
+else
+  { $as_echo "$as_me:${as_lineno-$LINENO}: result: no" >&5
+$as_echo "no" >&6; }
+fi
+
+
+  test -n "$PYTHON" && break
+done
+test -n "$PYTHON" || PYTHON="err"
+
+fi
+if test "$PYTHON" = "err"; then :
+  as_fn_error $? "No python interpreter found" "$LINENO" 5
 fi
 if echo "$PYTHON" | grep -q "^/"; then :
 
 else
-  PYTHON=`type -p "$PYTHON"`
+  # Extract the first word of "$PYTHON", so it can be a program name with args.
+set dummy $PYTHON; ac_word=$2
+{ $as_echo "$as_me:${as_lineno-$LINENO}: checking for $ac_word" >&5
+$as_echo_n "checking for $ac_word... " >&6; }
+if ${ac_cv_path_PYTHON+:} false; then :
+  $as_echo_n "(cached) " >&6
+else
+  case $PYTHON in
+  [\\/]* | ?:[\\/]*)
+  ac_cv_path_PYTHON="$PYTHON" # Let the user override the test with a path.
+  ;;
+  *)
+  as_save_IFS=$IFS; IFS=$PATH_SEPARATOR
+for as_dir in $PATH
+do
+  IFS=$as_save_IFS
+  test -z "$as_dir" && as_dir=.
+    for ac_exec_ext in '' $ac_executable_extensions; do
+  if as_fn_executable_p "$as_dir/$ac_word$ac_exec_ext"; then
+    ac_cv_path_PYTHON="$as_dir/$ac_word$ac_exec_ext"
+    $as_echo "$as_me:${as_lineno-$LINENO}: found $as_dir/$ac_word$ac_exec_ext" >&5
+    break 2
+  fi
+done
+  done
+IFS=$as_save_IFS
+
+  ;;
+esac
+fi
+PYTHON=$ac_cv_path_PYTHON
+if test -n "$PYTHON"; then
+  { $as_echo "$as_me:${as_lineno-$LINENO}: result: $PYTHON" >&5
+$as_echo "$PYTHON" >&6; }
+else
+  { $as_echo "$as_me:${as_lineno-$LINENO}: result: no" >&5
+$as_echo "no" >&6; }
+fi
+
+
 fi
 PYTHONPATH=$PYTHON
 PYTHON=`basename $PYTHONPATH`

tools/configure.ac

@@ -337,8 +337,9 @@ case "$host_os" in
   freebsd*) ;;
   *) AX_PATH_PROG_OR_FAIL([BASH], [bash]);;
 esac
-AS_IF([test -z "$PYTHON"], [PYTHON="python"])
-AS_IF([echo "$PYTHON" | grep -q "^/"], [], [PYTHON=`type -p "$PYTHON"`])
+AS_IF([test -z "$PYTHON"], [AC_CHECK_PROGS([PYTHON], [python python3 python2], err)])
+AS_IF([test "$PYTHON" = "err"], [AC_MSG_ERROR([No python interpreter found])])
+AS_IF([echo "$PYTHON" | grep -q "^/"], [], [AC_PATH_PROG([PYTHON], [$PYTHON])])
 PYTHONPATH=$PYTHON
 PYTHON=`basename $PYTHONPATH`