libtorrent: Make cpu popcount default (#10)

* libtorrent: Add CPU popcount support

This commit adds a native c-implementation to calculate the population count of the bitfield. It may be faster for CPUs which don't support SSE instructions. This is disabled by default and must be enabled by configuring with `--enable-cpu-popcount`.

* libtorrent: Make cpu popcount default

Benchmarked 30-50% reduction in CPU usage for uploading.

libtorrent/rak/algorithm.h

@@ -156,7 +156,6 @@ make_base(_InputIter __first, _InputIter __last, _Ftor __ftor) {
   return __base;
 }
 
-#if USE_CPU_POPCOUNT
 inline int countBit1Fast(unsigned int n) {
     n = (n & 0x55555555u) + ((n >> 1) & 0x55555555u);
     n = (n & 0x33333333u) + ((n >> 2) & 0x33333333u);
@@ -165,30 +164,10 @@ inline int countBit1Fast(unsigned int n) {
     n = (n & 0x0000ffffu) + ((n >>16) & 0x0000ffffu);
     return n;
 }
-#endif
 
 template<typename T>
 inline int popcount_wrapper(T t) {
-#if USE_CPU_POPCOUNT
   return countBit1Fast(t);
-#else	
-#if USE_BUILTIN_POPCOUNT
-  if (std::numeric_limits<T>::digits <= std::numeric_limits<unsigned int>::digits)
-    return __builtin_popcount(t);
-  else
-    return __builtin_popcountll(t);
-#else
-#error __builtin_popcount not found.
-  unsigned int count = 0;
-
-  while (t) {
-    count += t & 0x1;
-    t >> 1;
-  }
-
-  return count;
-#endif
-#endif
 }
 
 }