bug#58108: 29.0.50; [noverlay] Build error on MacOS

[Gerd Möllmann]

The branch currently does not build on macOS 12.6, since nil is reserved
in ObjC. See error below.  I'll try to fix that soon.

In file included from nsfns.m:41:
In file included from ./buffer.h:28:
./itree.h:57:24: error: expected member name or ';' after declaration specifiers
  struct interval_node nil;     /* The tree's version of NULL. */
                       ^
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/MacTypes.h:94:19:
 note: expanded from macro 'nil'
      #define nil __DARWIN_NULL
                  ^
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/sys/_types.h:52:25:
 note: expanded from macro '__DARWIN_NULL'
#define __DARWIN_NULL ((void *)0)
                        ^