scripts: stop dependencies weak-linking pipe2() and dup3()

pipe2() and dup3() are new in the 27.0 SDKs and carry
__API_AVAILABLE(macos(27.0), ios(27.0), ...). Both meson's has_function()
and autoconf's AC_CHECK_FUNCS declare their own prototype rather than
including the system header, so they never see that annotation and report
the functions present no matter the deployment target. The real call sites
do include <unistd.h>, so clang emits a weak import that is NULL on
anything older than 27 and the first call branches to zero.

That broke every VM start on iOS 26 and below: -[CSMain init] ->
g_main_context_new() -> g_wakeup_new() -> g_unix_open_pipe(), which calls
pipe2() when glib is configured with HAVE_PIPE2. glib, gio, gettext and
libusb all picked it up in every sysroot built with Xcode 27.

Tell both configure systems the functions are missing so the existing
pipe() + fcntl(FD_CLOEXEC) fallbacks are compiled in instead. Nothing
detects dup3() today, but cover it so the next dependency bump cannot
reintroduce the same crash.

Assisted-by: Claude:claude-opus-5

scripts/build_dependencies.sh

@@ -270,6 +270,12 @@ generate_meson_cross() {
     echo "# Automatically generated - do not modify" > $cross
     echo "[properties]" >> $cross
     echo "needs_exe_wrapper = true" >> $cross
+    # pipe2() and dup3() are new in the 27.0 SDKs. Meson's has_function() check declares
+    # its own prototype, so it never sees the availability attribute and reports them
+    # present no matter the deployment target. The real call sites do include the system
+    # header, which leaves a weak import that is NULL when running on an older OS.
+    echo "has_function_pipe2 = false" >> $cross
+    echo "has_function_dup3 = false" >> $cross
     echo "[built-in options]" >> $cross
     echo "c_args = [${CFLAGS:+$(meson_quote $CFLAGS)}]" >> $cross
     echo "cpp_args = [${CXXFLAGS:+$(meson_quote $CXXFLAGS)}]" >> $cross
@@ -1216,6 +1222,11 @@ export CXXFLAGS
 export OBJCFLAGS
 export LDFLAGS
 
+# Same reason as the has_function_* properties in the meson cross file: AC_CHECK_FUNCS
+# declares its own prototype and misses the 27.0 SDK availability attribute.
+export ac_cv_func_pipe2=no
+export ac_cv_func_dup3=no
+
 check_env
 echo "${GREEN}Starting build for ${PLATFORM_FAMILY_NAME} ${ARCH} [${NCPU} jobs]${NC}"