portability: don't use strtof

It is not available on HP-UX 11i.
Reported by Larkin Nickle <me@larbob.org>.
<https://lists.gnu.org/r/bug-bison/2021-09/msg00012.html>

Gnulib provides no replacement, but anyway we should be using doubles,
since difftime uses doubles.

* bootstrap.conf: We want portability on stdtod.
* src/counterexample.c: Use double, not float, for time measurements.

lib/.gitignore

@@ -391,6 +391,7 @@
 /stripslash.c
 /strndup.c
 /strnlen.c
+/strtod.c
 /strverscmp.c
 /sys
 /sys_ioctl.in.h