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.

bootstrap.conf

@@ -19,7 +19,7 @@
 gnulib_modules='
   argmatch array-list assert assure attribute
   bitsetv
-  calloc-posix close closeout config-h c-strcase
+  calloc-posix close closeout config-h c-strcase c-strtod
   configmake
   dirname
   error execute extensions
@@ -44,7 +44,7 @@ gnulib_modules='
   realloc-posix
   relocatable-prog relocatable-script
   rename
-  spawn-pipe stdbool stpcpy stpncpy strdup-posix strerror strtod strverscmp
+  spawn-pipe stdbool stpcpy stpncpy strdup-posix strerror strverscmp
   sys_ioctl
   termios
   timevar