Fix misconfiguration for macOS

`zutils.h` contains obsolete configurations for the macOS target that
are bug-prone and no longer exercised.
- The target OS conditional macros are misused for modern Apple
  platforms. `TARGET_OS_MAC` covers all Apple targets including iOS and
  so on. And it should not be checked with `#if defined` as they would
  always be defined (to either 1 or 0) on Apple platforms.
- The assumption that macOS does not have `fdopen`, or defines `fdopen`
  as a macro is no longer valid. The null definition in `zutils.h` would
  conflict with the macOS SDK and cause a compilation failure.

The problem hasn't manifested because the code path hasn't been
exercised for years because of the outdated `TARGET_OS` guard.
A recent extension in clang (llvm/llvm-project#74676)
would expose the issue and break zlib builds on Apple platforms.

Author: zixu-w

zutil.h

@@ -137,15 +137,11 @@ extern z_const char * const z_errmsg[10]; /* indexed by 2-zlib_error */
 #  endif
 #endif
 
-#if defined(MACOS) || defined(TARGET_OS_MAC)
+#if TARGET_OS_OSX
 #  define OS_CODE  7
 #  ifndef Z_SOLO
 #    if defined(__MWERKS__) && __dest_os != __be_os && __dest_os != __win32_os
 #      include <unix.h> /* for fdopen */
-#    else
-#      ifndef fdopen
-#        define fdopen(fd,mode) NULL /* No fdopen() */
-#      endif
 #    endif
 #  endif
 #endif