Use std::numeric_limits<> for typesafe INT_MAX/etc

(this fixes a Mac OS X gitian build error for 0.5.x)

src/util.cpp

@@ -144,7 +144,7 @@ uint64 GetRand(uint64 nMax)
 
     // The range of the random source must be a multiple of the modulus
     // to give every possible output value an equal possibility
-    uint64 nRange = (UINT64_MAX / nMax) * nMax;
+    uint64 nRange = (std::numeric_limits<uint64>::max() / nMax) * nMax;
     uint64 nRand = 0;
     do
         RAND_bytes((unsigned char*)&nRand, sizeof(nRand));