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.

bootstrap.conf

@@ -45,7 +45,7 @@ gnulib_modules='
   realloc-posix
   relocatable-prog relocatable-script
   rename
-  spawn-pipe stdbool stpcpy stpncpy strdup-posix strerror strverscmp
+  spawn-pipe stdbool stpcpy stpncpy strdup-posix strerror strtod strverscmp
   sys_ioctl
   termios
   timevar

lib/.gitignore

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

m4/.gitignore

@@ -216,6 +216,7 @@
 /strings_h.m4
 /strndup.m4
 /strnlen.m4
+/strtod.m4
 /strverscmp.m4
 /sys_ioctl_h.m4
 /sys_resource_h.m4

src/counterexample.c

@@ -57,12 +57,12 @@
 
 /** The time limit before printing an assurance message to the user to
  *  indicate that the search is still running. */
-#define ASSURANCE_LIMIT 2.0f
+#define ASSURANCE_LIMIT 2.0
 
 /* The time limit before giving up looking for unifying counterexample. */
-static float time_limit = 5.0f;
+static double time_limit = 5.0;
 
-#define CUMULATIVE_TIME_LIMIT 120.0f
+#define CUMULATIVE_TIME_LIMIT 120.0
 
 // This is the fastest way to get the tail node from the gl_list API.
 static gl_list_node_t
@@ -1164,7 +1164,7 @@ unifying_example (state_item_number itm1,
             }
           if (TIME_LIMIT_ENFORCED)
             {
-              float time_passed = difftime (time (NULL), start);
+              double time_passed = difftime (time (NULL), start);
               if (!assurance_printed && time_passed > ASSURANCE_LIMIT
                   && stage3result)
                 {
@@ -1218,7 +1218,7 @@ counterexample_init (void)
     if (cp)
       {
         char *end = NULL;
-        float v = strtof (cp, &end);
+        double v = strtod (cp, &end);
         if (*end == '\0' && errno == 0)
           time_limit = v;
       }