message: cmake: fix popcnt detection
  via GCC/Clang __POPCNT__ define
diff: # define CV_POPCNT_U64 _mm_popcnt_u64
  # endif
  # define CV_POPCNT_U32 _mm_popcnt_u32
  -#else
  +#elif defined(__POPCNT__)
  # include <popcntintrin.h>
  # if defined(__x86_64__)
  # define CV_POPCNT_U64 __builtin_popcountll
  # endif
  # define CV_POPCNT_U32 __builtin_popcount
  +#else
  +# error "__POPCNT__ is not defined by compiler"
  #endif
  int main()
  