build: fix --error-on-warn for macOS

XCode builds on macOS do not appear to inherit the `cflags` setting.

Signed-off-by: Richard Lau <riclau@uk.ibm.com>

PR-URL: nodejs#33357
Refs: nodejs#32685
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Jiawen Geng <technicalcute@gmail.com>

node.gyp

@@ -378,6 +378,9 @@
       'conditions': [
         [ 'error_on_warn=="true"', {
           'cflags': ['-Werror'],
+          'xcode_settings': {
+            'WARNING_CFLAGS': [ '-Werror' ],
+          },
         }],
         [ 'node_intermediate_lib_type=="static_library" and '
             'node_shared=="true" and OS=="aix"', {
@@ -757,6 +760,9 @@
       'conditions': [
         [ 'error_on_warn=="true"', {
           'cflags': ['-Werror'],
+          'xcode_settings': {
+            'WARNING_CFLAGS': [ '-Werror' ],
+          },
         }],
         [ 'node_builtin_modules_path!=""', {
           'defines': [ 'NODE_BUILTIN_MODULES_PATH="<(node_builtin_modules_path)"' ]