Improve string(RANDOM) default seed

The naive time(0) seed is unique only within one second.  Instead try to
read a real source of entropy and otherwise fall back to a combination
of the process id and high-resolution time.

Source/cmStringCommand.cxx

@@ -770,7 +770,7 @@ bool cmStringCommand
 
   static bool seeded = false;
   bool force_seed = false;
-  int seed = (int) time(NULL);
+  int seed = 0;
   int length = 5;
   const char cmStringCommandDefaultAlphabet[] = "qwertyuiopasdfghjklzxcvbnm"
     "QWERTYUIOPASDFGHJKLZXCVBNM"
@@ -825,7 +825,7 @@ bool cmStringCommand
   if (!seeded || force_seed)
     {
     seeded = true;
-    srand(seed);
+    srand(force_seed? seed : cmSystemTools::RandomSeed());
     }
 
   const char* alphaPtr = alphabet.c_str();