compile.h, utils.h: rename popcount

Rename `popcountl` to `popcntul` to avoid name conflicts with NetBSD
system headers.

Remove the unused `popcount` function.

Closes #502

Signed-off-by: Yuxuan Shui <yshuiv7@gmail.com>

src/utils.h

@@ -136,10 +136,6 @@ static inline int attr_const normalize_i_range(int i, int min, int max) {
 /// clamp `val` into interval [min, max]
 #define clamp(val, min, max) max2(min2(val, max), min)
 
-static inline int attr_const popcountl(unsigned long a) {
-	return __builtin_popcountl(a);
-}
-
 /**
  * Normalize a double value to a specific range.
  *