build: ignore deprecated-declaration warnings in objc++ macOS code

These come from GUI code, and haven't/aren't being fixed, see discussion
in bitcoin-core/gui#112. For now, just ignore
them entirely. Note that this only applies to ObjCXX code, so will not
hide any relevant warnings coming from C or CXX code (and they would be
unlikely in any case).

Alternative to #29362, which disables all compiler warnings, for macOS
builds in the CI.

Relevant output:
```bash
qt/macnotificationhandler.mm:27:9: warning: 'NSUserNotification' is deprecated: first deprecated in macOS 11.0 - All NSUserNotifications API should be replaced with UserNotifications.frameworks API [-Wdeprecated-declarations]
        NSUserNotification* userNotification = [[NSUserNotification alloc] init];
        ^
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/System/Library/Frameworks/Foundation.framework/Headers/NSUserNotification.h:24:12: note: 'NSUserNotification' has been explicitly marked deprecated here
@interface NSUserNotification : NSObject <NSCopying> {
           ^
qt/macnotificationhandler.mm:27:50: warning: 'NSUserNotification' is deprecated: first deprecated in macOS 11.0 - All NSUserNotifications API should be replaced with UserNotifications.frameworks API [-Wdeprecated-declarations]
        NSUserNotification* userNotification = [[NSUserNotification alloc] init];
                                                 ^
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/System/Library/Frameworks/Foundation.framework/Headers/NSUserNotification.h:24:12: note: 'NSUserNotification' has been explicitly marked deprecated here
@interface NSUserNotification : NSObject <NSCopying> {
           ^
qt/macnotificationhandler.mm:30:11: warning: 'NSUserNotificationCenter' is deprecated: first deprecated in macOS 11.0 - All NSUserNotifications API should be replaced with UserNotifications.frameworks API [-Wdeprecated-declarations]
        [[NSUserNotificationCenter defaultUserNotificationCenter] deliverNotification: userNotification];
          ^
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/System/Library/Frameworks/Foundation.framework/Headers/NSUserNotification.h:118:12: note: 'NSUserNotificationCenter' has been explicitly marked deprecated here
@interface NSUserNotificationCenter : NSObject {
           ^
3 warnings generated.
```

configure.ac

@@ -780,7 +780,10 @@ case $host in
 
      AX_CHECK_LINK_FLAG([-Wl,-headerpad_max_install_names], [CORE_LDFLAGS="$CORE_LDFLAGS -Wl,-headerpad_max_install_names"], [], [$LDFLAG_WERROR])
      CORE_CPPFLAGS="$CORE_CPPFLAGS -DMAC_OSX -DOBJC_OLD_DISPATCH_PROTOTYPES=0"
-     OBJCXXFLAGS="$CXXFLAGS"
+
+     dnl ignore deprecated-declerations warnings coming from objcxx code
+     dnl "'NSUserNotificationCenter' is deprecated: first deprecated in macOS 11.0".
+     OBJCXXFLAGS="$CXXFLAGS -Wno-deprecated-declarations"
      ;;
    *android*)
      dnl make sure android stays above linux for hosts like *linux-android*