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.

m4/.gitignore

@@ -7,6 +7,7 @@
 /asm-underscore.m4
 /assert.m4
 /builtin-expect.m4
+/c-strtod.m4
 /calloc.m4
 /canonicalize.m4
 /chdir-long.m4
@@ -83,6 +84,7 @@
 /include_next.m4
 /inline.m4
 /intdiv0.m4
+/intl-thread-locale.m4
 /intl.m4
 /intldir.m4
 /intlmacosx.m4