Merge bitcoin#20586: Fix Windows build with --enable-werror

BACKPORT NOTE:
Missing changes in src/fs.cpp are removed in bitcoin#20744 which is already backported

b367745 ci: Make Cirrus CI Windows build with --enable-werror (Hennadii Stepanov)
c713bb2 Fix Windows build with --enable-werror on Ubuntu Focal (Hennadii Stepanov)

Pull request description:

  This PR makes possible to cross-compile Windows build with `--enable-werror --enable-suppress-external-warnings`.
  Some problems are fixed, others are silenced.

  Also `--enable-werror` is enabled for Cirrus CI Windows build (the last one on Cirrus CI without `--enable-werror`).

ACKs for top commit:
  practicalswift:
    cr ACK b367745: patch looks correct
  laanwj:
    Code review ACK b367745
  vasild:
    ACK b367745
  jarolrod:
    ACK b367745

Tree-SHA512: 64f5c99b7dad4c0efce80cd45d7074f275bd8411235dc9e0841287bdab64b812c6f8f9d632c35531d0b8210148531f53aaaac77be7699b29d2d6aaae304dbee0

ci/test/00_setup_env_win64.sh

@@ -15,7 +15,3 @@ export RUN_SECURITY_TESTS="false"
 export GOAL="deploy"
 export BITCOIN_CONFIG="--enable-gui --enable-reduce-exports --disable-miner --without-boost-process"
 export DIRECT_WINE_EXEC_TESTS=true
-
-# Compiler for MinGW-w64 causes false -Wreturn-type warning.
-# See https://sourceforge.net/p/mingw-w64/bugs/306/
-export NO_WERROR=1

configure.ac

@@ -462,7 +462,13 @@ if test "x$enable_werror" = "xyes"; then
   AX_CHECK_COMPILE_FLAG([-Werror=range-loop-analysis],[ERROR_CXXFLAGS="$ERROR_CXXFLAGS -Werror=range-loop-analysis"],,[[$CXXFLAG_WERROR]])
   AX_CHECK_COMPILE_FLAG([-Werror=unused-variable],[ERROR_CXXFLAGS="$ERROR_CXXFLAGS -Werror=unused-variable"],,[[$CXXFLAG_WERROR]])
   AX_CHECK_COMPILE_FLAG([-Werror=date-time],[ERROR_CXXFLAGS="$ERROR_CXXFLAGS -Werror=date-time"],,[[$CXXFLAG_WERROR]])
-  AX_CHECK_COMPILE_FLAG([-Werror=return-type],[ERROR_CXXFLAGS="$ERROR_CXXFLAGS -Werror=return-type"],,[[$CXXFLAG_WERROR]])
+
+  dnl -Wreturn-type is broken in GCC for MinGW-w64.
+  dnl https://sourceforge.net/p/mingw-w64/bugs/306/
+  AX_CHECK_COMPILE_FLAG([-Werror=return-type], [ERROR_CXXFLAGS="$ERROR_CXXFLAGS -Werror=return-type"], [], [$CXXFLAG_WERROR],
+                        [AC_LANG_SOURCE([[#include <cassert>
+                                          int f(){ assert(false); }]])])
+
   AX_CHECK_COMPILE_FLAG([-Werror=conditional-uninitialized],[ERROR_CXXFLAGS="$ERROR_CXXFLAGS -Werror=conditional-uninitialized"],,[[$CXXFLAG_WERROR]])
   AX_CHECK_COMPILE_FLAG([-Werror=sign-compare],[ERROR_CXXFLAGS="$ERROR_CXXFLAGS -Werror=sign-compare"],,[[$CXXFLAG_WERROR]])
   dnl -Wsuggest-override is broken with GCC before 9.2

src/qt/winshutdownmonitor.h

@@ -17,7 +17,7 @@ class WinShutdownMonitor : public QAbstractNativeEventFilter
 {
 public:
     /** Implements QAbstractNativeEventFilter interface for processing Windows messages */
-    bool nativeEventFilter(const QByteArray &eventType, void *pMessage, long *pnResult);
+    bool nativeEventFilter(const QByteArray &eventType, void *pMessage, long *pnResult) override;
 
     /** Register the reason for blocking shutdown on Windows to allow clean client exit */
     static void registerShutdownBlockReason(const QString& strReason, const HWND& mainWinId);