ci: document that -Wreturn-type has been fixed upstream (Windows)

`noreturn` attributes have been added to the mingw-w64 headers, meaning
that from 11.0.0 onwards, you'll no-longer see `-Wreturn-type` warnings
when using assert(false):
1690994f51.

Add -Wno-return-type to the Windows CI, where is should have been all
along, and document why it's required. This can be dropped when we are
using the fixed version of the mingw-w64 headers there.

Drop the -Werror -Wno-return-type special case from our build system.
-Wreturn-type is on by default in Clang and GCC.

ci/test/00_setup_env_win64.sh

@@ -13,4 +13,7 @@ export DPKG_ADD_ARCH="i386"
 export PACKAGES="nsis g++-mingw-w64-x86-64-posix wine-binfmt wine64 wine32 file"
 export RUN_FUNCTIONAL_TESTS=false
 export GOAL="deploy"
-export BITCOIN_CONFIG="--enable-reduce-exports --enable-external-signer --disable-gui-tests"
+# Prior to 11.0.0, the mingw-w64 headers were missing noreturn attributes, causing warnings when
+# cross-compiling for Windows. https://sourceforge.net/p/mingw-w64/bugs/306/
+# https://github.com/mingw-w64/mingw-w64/commit/1690994f515910a31b9fb7c7bd3a52d4ba987abe
+export BITCOIN_CONFIG="--enable-reduce-exports --enable-external-signer --disable-gui-tests CXXFLAGS=-Wno-return-type"

configure.ac

@@ -427,12 +427,6 @@ if test "$enable_werror" = "yes"; then
     AC_MSG_ERROR([enable-werror set but -Werror is not usable])
   fi
   ERROR_CXXFLAGS=$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="$ERROR_CXXFLAGS -Wno-error=return-type"], [$CXXFLAG_WERROR],
-                        [AC_LANG_SOURCE([[#include <cassert>
-                                          int f(){ assert(false); }]])])
 fi
 
 if test "$CXXFLAGS_overridden" = "no"; then