Correctly describe POPCNT compile

SSE4.2 has nothing to do with POPCNT. We must dispell this myth, because
Stockfish is a reference and many will copy this mistake if they see it in Stockfish:
* SSE is an SIMD instruction set, relative to vectorization (using special 128-bit registers).
* POPCNT/LZCNT work on normal registers (eg. AL, AX, EAX, RAX).

The confusion comes from the fact that, in the Intel product line, it just
so happens that SSE4.2 and POPCNT/LZCNT came out at the same time. But this
is not true for AMD. For example, all AMD Pheniom II have SSE3 but no
POPCNT/LZCNT, and that is why the modern compile uses  -msse3 -popcnt and not -msse4.2.

No functional change.

Resolves #86

src/misc.cpp

@@ -115,7 +115,7 @@ const string engine_info(bool to_uci) {
   }
 
   ss << (Is64Bit ? " 64" : "")
-     << (HasPext ? " BMI2" : (HasPopCnt ? " SSE4.2" : ""))
+     << (HasPext ? " BMI2" : (HasPopCnt ? " POPCNT" : ""))
      << (to_uci  ? "\nid author ": " by ")
      << "Tord Romstad, Marco Costalba and Joona Kiiski";