(qb) Prefer system zlib on macOS

The vendored zlib in deps/libz (zlib 1.3) fails to compile against
recent macOS SDKs (tested with macOS 26.x / Xcode CLT 21). Its
zutil.h defines fdopen() as a macro expanding to NULL, which
collides with the fdopen declaration in <stdio.h>:

    error: expected identifier or '('
    FILE *fdopen(int, const char *) __DARWIN_ALIAS_STARTING(...);

Upstream zlib fixed this in 1.3.1, but rather than bump the bundled
copy for one platform, default HAVE_BUILTINZLIB to "no" on Darwin so
the system libz is used. macOS has shipped a working libz for years
and it's what the vast majority of Mac builds end up linking against
anyway.

Users who want the vendored copy can still pass --enable-builtinzlib
explicitly; the override only triggers when HAVE_BUILTINZLIB is still
at its default "auto" value.

Author: LibretroAdmin

qb/config.libs.sh

@@ -375,6 +375,11 @@ if [ "$OS" = 'Darwin' ]; then
    HAVE_X11=no # X11 breaks on recent OSXes even if present.
    HAVE_SDL=no
    HAVE_SW2=no
+   # Prefer the system zlib on macOS. The vendored copy in deps/libz
+   # (zlib 1.3) no longer compiles against recent macOS SDKs because
+   # its zutil.h defines fdopen() as a macro that collides with the
+   # fdopen declaration in <stdio.h>. The system libz works fine.
+   [ "$HAVE_BUILTINZLIB" = 'auto' ] && HAVE_BUILTINZLIB=no
 else
    check_lib '' AL -lopenal alcOpenDevice
 fi