configure.ac: fix the check for xmlto availability
The same $xmlto variable is used both in AC_ARG_ENABLE and
AC_CHECK_PROG, but the latter is not setting a value to it when the
program is not found.
These two facts result in the "yes" value from the AC_ARG_ENABLE macro
to be still kept in the variable when the program is not found by
AC_CHECK_PROG, causing USE_XMLTO to be always set, finally resulting in
a build failure in case the xmlto program is not actually in the PATH.
As possible fix could have been to set "no" as a value in AC_CHECK_PROG
when program is not found.
However using two separate variables is more explicit, so fix the issue
this way.
Signed-off-by: Antonio Ospite <ao2@ao2.it> Signed-off-by: Takashi Iwai <tiwai@suse.de>