Clang has __popcnt for ARM

[zcbenz]

When building on Windows with clang for ARM targets, __popcnt is defined.

[Keitagit-kun]

I'm building on Windows with mingw-w64 x86_64-7.2.0 for windows and __popcnt is also missing. So, maybe cover all cases like that:

#if !defined __popcnt
	static unsigned int __popcnt(unsigned int x) {
	  unsigned int c = 0;
	  for (; x; ++c) {
		x &= x - 1;
	  }
	  return c;
	}
#endif

"static" - so it wont interfere with ngtcp2...

[tatsuhiro-t]

It would be nice to add function detection to configure and cmake scripts to handle this generally.

[tatsuhiro-t]

Thank you for PR. Merged now.