Implement a simple xorshift 32bit random number generator

Avoid relying on the system random number generator for our random
shuffle, so that the same seed gives the same results on all systems.
This generator doesn't need to be amazing, just pretty good, so don't
bother with xorshift* or xorshift+, etc.

* src/makeint.h: Declare make_seed() and make_rand().
* src/misc.c (make_seed): Set the seed value for the RNG.
(make_rand): Return the next random number.  If the seed was not set
initialize it first.
* src/shuffle.c (shuffle_set_mode): If we don't get a seed from the
user just leave it unset (0).
(shuffle_deps_recursive): Use make_seed() not srand().
(random_shuffle_array): Use make_rand() not rand().

src/makeint.h

@@ -537,6 +537,8 @@ void perror_with_name (const char *, const char *);
 unsigned int make_toui (const char*, const char**);
 char *make_lltoa (long long, char *);
 char *make_ulltoa (unsigned long long, char *);
+void make_seed (unsigned int);
+unsigned int make_rand ();
 pid_t make_pid ();
 void *xmalloc (size_t);
 void *xcalloc (size_t);

src/misc.c

@@ -78,6 +78,35 @@ make_ulltoa (unsigned long long val, char *buf)
   return buf;
 }
 
+/* Simple random number generator, for use with shuffle.
+   This doesn't need to be truly random, just pretty random.  Use our own
+   implementation rather than relying on the C runtime's rand() so we always
+   get the same results for a given seed, regardless of OS.  */
+
+static unsigned int mk_state = 0;
+
+void
+make_seed(unsigned int seed)
+{
+  mk_state = seed;
+}
+
+unsigned int
+make_rand()
+{
+  /* mk_state must never be 0.  */
+  if (mk_state == 0) {
+    mk_state = (unsigned int)(time (NULL) ^ make_pid ()) + 1;
+  }
+
+  /* A simple xorshift RNG.  */
+  mk_state ^= mk_state << 13;
+  mk_state ^= mk_state >> 17;
+  mk_state ^= mk_state << 5;
+
+  return mk_state;
+}
+
 /* Compare strings *S1 and *S2.
    Return negative if the first is less, positive if it is greater,
    zero if they are equal.  */

src/shuffle.c

@@ -82,9 +82,7 @@ shuffle_set_mode (const char *cmdarg)
     }
   else
     {
-      if (strcasecmp (cmdarg, "random") == 0)
-        config.seed = (unsigned int) (time (NULL) ^ make_pid ());
-      else
+      if (strcasecmp (cmdarg, "random") != 0)
         {
           /* Assume explicit seed.  */
           const char *err;
@@ -109,7 +107,7 @@ random_shuffle_array (void **a, size_t len)
       void *t;
 
       /* Pick random element and swap. */
-      unsigned int j = rand () % len;
+      unsigned int j = make_rand () % len;
       if (i == j)
         continue;
 
@@ -227,7 +225,7 @@ shuffle_deps_recursive (struct dep *deps)
 
   /* Set specific seed at the top level of recursion.  */
   if (config.mode == sm_random)
-    srand (config.seed);
+    make_seed (config.seed);
 
   shuffle_deps (deps);