build: Use __builtin_popcount() if available.

include/private/vkd3d_common.h

@@ -39,9 +39,13 @@ static inline size_t align(size_t addr, size_t alignment)
 
 static inline unsigned int vkd3d_popcount(unsigned int v)
 {
+#ifdef HAVE_BUILTIN_POPCOUNT
+    return __builtin_popcount(v);
+#else
     v -= (v >> 1) & 0x55555555;
     v = (v & 0x33333333) + ((v >> 2) & 0x33333333);
     return (((v + (v >> 4)) & 0x0f0f0f0f) * 0x01010101) >> 24;
+#endif
 }
 
 #ifndef _WIN32