seed mt19937 with random data

time(NULL) is a poor seed. It is quite predictable and
multiple instance starting in the same second will get the same seed
and therefore the same pseudo random number sequence

Use std::random_device, witch is meant to provide 'true' random
data.. mix time(NULL) just in case the std implementation is crappy.

PS: sadly std::random_device.entropy() cannot be relied on
as clang and gcc are known to return 0 despite their random_device
being non-deterministic, hence the prophylactic systematic
mixing with time(null)

Change-Id: I44dab9970f8f0388dc1c99e9816d49d1afbecf18
Reviewed-on: https://gerrit.libreoffice.org/15591
Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Michael Stahl <mstahl@redhat.com>

comphelper/source/misc/random.cxx

@@ -37,12 +37,13 @@ struct RandomNumberGenerator
     STD_RNG_ALGO global_rng;
     RandomNumberGenerator()
     {
+        std::random_device rd;
         // initialises the state of the global random number generator
         // should only be called once.
         // (note, a few std::variate_generator<> (like normal) have their
         // own state which would need a reset as well to guarantee identical
         // sequence of numbers, e.g. via myrand.distribution().reset())
-        global_rng.seed(time(NULL));
+        global_rng.seed(rd() ^ time(nullptr));
     }
 };