Fix errors: passing argument 2 of ‘signal’ from incompatible pointer …

[Rtoax]

Fix compile errors on Fedora 42

For example:

$ ./Run -c 1 dhry2reg whetstone-double
gcc -o pgms/arithoh -Wall -pedantic -O3 -ffast-math -march=native -mtune=native -I ./src -DTIME -Darithoh src/arith.c
In file included from src/arith.c:34:
src/timeit.c: In function ‘wake_me’:
src/timeit.c:32:6: warning: old-style function definition [-Wold-style-definition]
   32 | void wake_me(seconds, func)
      |      ^~~~~~~
src/timeit.c:37:25: error: passing argument 2 of ‘signal’ from incompatible pointer type [-Wincompatible-pointer-types]
   37 |         signal(SIGALRM, func);
      |                         ^~~~
      |                         |
      |                         void (*)(void)
In file included from src/timeit.c:29:
/usr/include/signal.h:88:57: note: expected ‘__sighandler_t’ {aka ‘void (*)(int)’} but argument is of type ‘void (*)(void)’
   88 | extern __sighandler_t signal (int __sig, __sighandler_t __handler)
      |                                          ~~~~~~~~~~~~~~~^~~~~~~~~
/usr/include/signal.h:72:16: note: ‘__sighandler_t’ declared here
   72 | typedef void (*__sighandler_t) (int);
      |                ^~~~~~~~~~~~~~

[loreb]

It's not just fedora, it's anything with gcc >= 15 (I think); make CC='gcc -std=gnu99' or similar is a quick&dirty workaround