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.

lib/.gitignore

@@ -40,6 +40,8 @@
 /c-strcasecmp.c
 /c-strcaseeq.h
 /c-strncasecmp.c
+/c-strtod.c
+/c-strtod.h
 /calloc.c
 /canonicalize-lgpl.c
 /canonicalize.c