Merge bitcoin/bitcoin#28092: ci: document that -Wreturn-type has been fixed upstream (mingw-w64)

08eb5f1 ci: document that -Wreturn-type has been fixed upstream (Windows) (fanquake)

Pull request description:

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

  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.

  The new mingw-w64 header behaviour can be checked on Ubuntu mantic, [which ships with 11.0.0](https://packages.ubuntu.com/mantic/mingw-w64), using:
  ```cpp
  #include <cassert>

  int f(){ assert(false); }

  int main() {
  return 0;
  }
  ```

  On Mantic (with 11.0.0):
  ```bash
  x86_64-w64-mingw32-g++ test.cpp -Wreturn-type
  # nada
  ```

  On Lunar ([with 10.0.0](https://packages.ubuntu.com/lunar/mingw-w64)):
  ```bash
  x86_64-w64-mingw32-g++ test.cpp -Wreturn-type
  test.cpp: In function 'int f()':
  test.cpp:3:25: warning: no return statement in function returning non-void [-Wreturn-type]
      3 | int f(){ assert(false); }
        |                         ^
  ```

ACKs for top commit:
  TheCharlatan:
    ACK 08eb5f1

Tree-SHA512: 9cd4310a96abd87bf8ceb37949ad0259fe4adee3367c604f4c4ad521a0cf09bdcc5dd305db19a0f45ce74c85178b0d739e2fca5ad0fc841ac935523a23b28a7f

Author: fanquake

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