Use rand not random for creating GC root indirect links

I don't think fewer bits matters for this, and `rand` but not `random`
is available on Windows.

src/libmain/shared.cc

@@ -173,12 +173,13 @@ void initNix()
        everybody. */
     umask(0022);
 
-#ifndef _WIN32
     /* Initialise the PRNG. */
     struct timeval tv;
     gettimeofday(&tv, 0);
+#ifndef _WIN32
     srandom(tv.tv_usec);
 #endif
+    srand(tv.tv_usec);
 
 
 }