Merge bitcoin/bitcoin#29577: build: ignore deprecated-declarations warnings in objc++ macOS code

8b7630c build: ignore deprecated-declaration warnings in objc++ macOS code (fanquake)
bd8f035 build: Add missed definition for `AM_OBJCXXFLAGS` (Hennadii Stepanov)

Pull request description:

  These come from GUI code, and haven't/aren't being fixed, see discussion in #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 bitcoin/bitcoin#29362 (which disables all compiler warnings, for macOS builds in the CI). This PR includes one commit from that PR.

  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.
  ```

ACKs for top commit:
  hebasto:
    re-ACK 8b7630c.

Tree-SHA512: 2f1fec97d9aa46aa23989d9fb283fc574dff9dc8f44847bb273b8fcf942f56f64c6d93dfcd7af8fbb52bf152e0fe76818118ce416d8cec5de852c32b6697a243

Author: fanquake

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-declarations 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*

src/Makefile.am

@@ -10,6 +10,7 @@ DIST_SUBDIRS = secp256k1
 
 AM_LDFLAGS = $(LIBTOOL_LDFLAGS) $(HARDENED_LDFLAGS) $(GPROF_LDFLAGS) $(SANITIZER_LDFLAGS) $(CORE_LDFLAGS)
 AM_CXXFLAGS = $(CORE_CXXFLAGS) $(DEBUG_CXXFLAGS) $(HARDENED_CXXFLAGS) $(WARN_CXXFLAGS) $(NOWARN_CXXFLAGS) $(ERROR_CXXFLAGS) $(GPROF_CXXFLAGS) $(SANITIZER_CXXFLAGS)
+AM_OBJCXXFLAGS = $(AM_CXXFLAGS)
 AM_CPPFLAGS = $(DEBUG_CPPFLAGS) $(HARDENED_CPPFLAGS) $(CORE_CPPFLAGS)
 AM_LIBTOOLFLAGS = --preserve-dup-deps
 PTHREAD_FLAGS = $(PTHREAD_CFLAGS) $(PTHREAD_LIBS)