[macOS] v. 5.9.7 broken on macOS < 10.13: `if_dl.h: error: unknown type name 'u_char'; did you mean 'char'?` about psutil HOT 1 CLOSED

Sergey also filed this with MacPorts where he noted that it builds fine on macOS 10.13 and later, but not on 10.12 and earlier. This suggested to me that maybe some system header that is automatically included on 10.13 and later that provides the definition of u_char needed to be included manually.

I found that old macOS system headers have a bug that net/if_df.h does not include sys/types.h, but net/if.h does, so including that headers before, rather than after, net/if_dl.h works around the problem; I submitted a PR for that.

More generally, following the convention of including system headers in alphabetical order should minimize these kinds of problems.

There are further build failures on old macOS systems after fixing this. I'll investigate and report that separately.

from psutil.