Merge bitcoin#18738: build: Suppress -Wdeprecated-copy warnings

0c63f80 build: Suppress -Wdeprecated-copy warnings (Hennadii Stepanov)

Pull request description:

  Tomorrow, on Apr 23 the Ubuntu 20.04 release is expected. It packaged with Qt 5.12 LTS that has a nasty peculiarity to cause modern compilers, including Clang 10.0 and GCC 9.3, to emit spammy `-Wdeprecated-copy` warnings (bitcoin#15822, bitcoin#18419).

  This PR suppress such warnings _temporarily_, until the [upstream is fixed](https://codereview.qt-project.org/c/qt/qtbase/+/272258).

  Here are some affected systems (with system packages):
  - Ubuntu 20.04 LTS + Qt 5.12.8 LTS + { Clang 10.0 | GCC 9.3 }
  - Fedora 32 + Qt 5.13.2 + Clang 10.0

  Reference: [QTBUG-75210](https://bugreports.qt.io/browse/QTBUG-75210)

  Also see **fanquake**'s [comment](bitcoin#18738 (comment)).

ACKs for top commit:
  MarcoFalke:
    ACK 0c63f80 seems fine to disable this warning for the 0.21.0 release temporarily and then enable it for 0.22.0, when boost is removed.
  fanquake:
    ACK 0c63f80 - I think it's ok to suppress these for now, given that `-Wdeprecated-copy` is enabled (via `-Wextra`) in GCC 9 and Clang 10. The Qt output is pretty noisy, and there's a few warnings from Boost as well.

Tree-SHA512: 7064a3272bc9eae00b73a16c421ac58be148f374cbef87320e8f092f52761f6e98166eff60346b70867f8a69a9698a79455dc16b42d92f8fbe7c56519571ac08

configure.ac

@@ -410,6 +410,7 @@ if test "x$CXXFLAGS_overridden" = "xno"; then
   AX_CHECK_COMPILE_FLAG([-Wunused-local-typedef],[NOWARN_CXXFLAGS="$NOWARN_CXXFLAGS -Wno-unused-local-typedef"],,[[$CXXFLAG_WERROR]])
   AX_CHECK_COMPILE_FLAG([-Wdeprecated-register],[NOWARN_CXXFLAGS="$NOWARN_CXXFLAGS -Wno-deprecated-register"],,[[$CXXFLAG_WERROR]])
   AX_CHECK_COMPILE_FLAG([-Wimplicit-fallthrough],[NOWARN_CXXFLAGS="$NOWARN_CXXFLAGS -Wno-implicit-fallthrough"],,[[$CXXFLAG_WERROR]])
+  AX_CHECK_COMPILE_FLAG([-Wdeprecated-copy],[NOWARN_CXXFLAGS="$NOWARN_CXXFLAGS -Wno-deprecated-copy"],,[[$CXXFLAG_WERROR]])
 fi
 
 enable_hwcrc32=no