Merge bitcoin/bitcoin#22258: build: Disable deprecated-copy warning only when external warnings are enabled

1111457 build: Disable deprecated-copy warning only when external warnings are enabled (MarcoFalke)

Pull request description:

  Fixes bitcoin/bitcoin#18967

  Alternative to bitcoin/bitcoin#22240

ACKs for top commit:
  fanquake:
    tACK 1111457

Tree-SHA512: 0fc826f26ebbeab662fa7eed2a5cc1630c6c4e612deb91734885fc8bae0352be657ec48ae94ff55a984ac36d27b95cea8d947cc5cf408231d56addecf79db83f

Author: fanquake

configure.ac

@@ -475,7 +475,9 @@ if test "x$CXXFLAGS_overridden" = "xno"; then
   AX_CHECK_COMPILE_FLAG([-Wself-assign],[NOWARN_CXXFLAGS="$NOWARN_CXXFLAGS -Wno-self-assign"],,[[$CXXFLAG_WERROR]])
   AX_CHECK_COMPILE_FLAG([-Wunused-local-typedef],[NOWARN_CXXFLAGS="$NOWARN_CXXFLAGS -Wno-unused-local-typedef"],,[[$CXXFLAG_WERROR]])
   AX_CHECK_COMPILE_FLAG([-Wimplicit-fallthrough],[NOWARN_CXXFLAGS="$NOWARN_CXXFLAGS -Wno-implicit-fallthrough"],,[[$CXXFLAG_WERROR]])
-  AX_CHECK_COMPILE_FLAG([-Wdeprecated-copy],[NOWARN_CXXFLAGS="$NOWARN_CXXFLAGS -Wno-deprecated-copy"],,[[$CXXFLAG_WERROR]])
+  if test x$suppress_external_warnings != xyes ; then
+    AX_CHECK_COMPILE_FLAG([-Wdeprecated-copy],[NOWARN_CXXFLAGS="$NOWARN_CXXFLAGS -Wno-deprecated-copy"],,[[$CXXFLAG_WERROR]])
+  fi
 fi
 
 dnl Don't allow extended (non-ASCII) symbols in identifiers. This is easier for code review.