cex: provide the user with a means to change the timeout

Reported by Frank Heckenbach.
https://lists.gnu.org/r/bug-bison/2022-07/msg00011.html

* bootstrap.conf: Use c_strtod, so that even in French locales "1.5"
is accepted, instead of "1,5".
* src/counterexample.c, src/state-item.c: Use xtime_t instead of
time_t, so that accuracy goes from seconds to nanoseconds.
( counterexample_init): Depend on cex.timeout rather than
$TIME_LIMIT.
* doc/bison.texi (%define Summary): Document cex.timeout.

tests/conflicts.at

@@ -1958,7 +1958,7 @@ empty_c3: %prec 'c' ;
 
 AT_BISON_CHECK([[--trace=cex -fcaret --report=all -o input.c input.y]], 0, [],
 [[bison (GNU Bison) ]AT_PACKAGE_VERSION[
-init: 0.000000
+state_items_init: 0.000s
 # state items: 26
 State 0:
     0 $accept: . start $end