Fix gitstatus build failure with Apple Clang 17 (Xcode 16+)

[alejandroqh]

Apple Clang 17 predefines TARGET_OS_MAC=1, which activates an ancient classic Mac OS code path in the bundled zlib. The fdopen macro replaces the system _stdio.h declaration of fdopen() with NULL, producing a parse error.

Error:

  _stdio.h:322:7: error: expected identifier or '('
    FILE *fdopen(int, const char *) ...

  zutil.h:140:33: note: expanded from macro 'fdopen'
    #define fdopen(fd,mode) NULL /* No fdopen() */

Fix: use system zlib instead of the bundled one.

• Add -lz to gitstatus_ldlibs to link against system zlib
• Add -DUSE_BUNDLED_ZLIB=OFF to libgit2 cmake flags