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.

doc/bison.texi

@@ -6894,6 +6894,23 @@ Introduced in Bison 3.0.3.
 @c api.value.union.name
 
 
+@c ================================================== cex.timeout
+
+@deffn Directive {%define cex.timeout} @var{duration}
+
+@itemize @bullet
+@item Language(s): all
+
+@item Purpose:
+Define the time limit for finding unifying counterexamples.
+
+@item Accepted Values: duration in seconds, e.g., @samp{1}, @samp{0.5}.
+
+@item Default Value: 5
+@end itemize
+@end deffn
+
+
 @c ================================================== lr.default-reduction
 
 @deffn Directive {%define lr.default-reduction} @var{when}