Merge bitcoin#30236: build: re-enable deprecated warning copy

c3a5e8a build: re-enable deprecated warning copy (Cory Fields)

Pull request description:

  Noticed while looking at the `-wno-*` flags in bitcoin#30235.

  This was disabled in bitcoin#18738 due to the combo of old gcc and qt. We no longer support the affected gcc, and the old qt should no longer be relevant to us anyway.

  See old fixes in:
  https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88136
  and
  https://bugreports.qt.io/browse/QTBUG-75210
  and
  https://codereview.qt-project.org/c/qt/qtbase/+/245434

ACKs for top commit:
  maflcko:
    ACK c3a5e8a
  fanquake:
    ACK c3a5e8a - this is in `-Wextra` for Clang and GCC.

Tree-SHA512: bd008dc50134d15ca3bb0c4f78d910db5b7a0ee98b04c159122a6f13a24b18827806492f053293d9cc1f1528ba60dea6d9ed31a366f63840ccb7c55f002d263b

Author: fanquake

configure.ac

@@ -417,9 +417,6 @@ dnl unknown options if any other warning is produced. Test the -Wfoo case, and
 dnl set the -Wno-foo case if it works.
 AX_CHECK_COMPILE_FLAG([-Wunused-parameter], [NOWARN_CXXFLAGS="$NOWARN_CXXFLAGS -Wno-unused-parameter"], [], [$CXXFLAG_WERROR])
 AX_CHECK_COMPILE_FLAG([-Wself-assign], [NOWARN_CXXFLAGS="$NOWARN_CXXFLAGS -Wno-self-assign"], [], [$CXXFLAG_WERROR])
-if test "$suppress_external_warnings" != "yes" ; then
-  AX_CHECK_COMPILE_FLAG([-Wdeprecated-copy], [NOWARN_CXXFLAGS="$NOWARN_CXXFLAGS -Wno-deprecated-copy"], [], [$CXXFLAG_WERROR])
-fi
 
 dnl Don't allow extended (non-ASCII) symbols in identifiers. This is easier for code review.
 AX_CHECK_COMPILE_FLAG([-fno-extended-identifiers], [CORE_CXXFLAGS="$CORE_CXXFLAGS -fno-extended-identifiers"], [], [$CXXFLAG_WERROR])