Merge bitcoin#24131: build, qt: Fix Windows cross-compiling with Qt 5.15

BACKPORT NOTE:
extra changes in build-windows.md due to missing bitcoin#11438

9796dca doc: Install only "-posix" MinGW compiler when possible (Hennadii Stepanov)
0bbae23 ci: Drop no longer needed `update-alternatives` (Hennadii Stepanov)
01d1845 build, qt: Specify QMAKE_CXX explicitly (Hennadii Stepanov)

Pull request description:

  While changes introduced in bitcoin#22093 worked fine with Qt 5.12, after bumping Qt up to 5.15 the cross-compiling of `qt` package for Windows fails with `error: ‘mutex’ in namespace ‘std’ does not name a type`.

  The first commit fixes this bug.

  The second commit cleans up a related CI script.

  The third commit improves related docs (see bitcoin#22093 (comment)).

ACKs for top commit:
  prusnak:
    ACK 9796dca

Tree-SHA512: 0dc46c5dfab85bd6d2901052cd630e86f9b4e09c08ef87136b44ddecb1783cdf3cd0a6e67b95ac7a78da24cd7adedc88745f61f9a8d9993fbff26d33bf88d874

Author: fanquake

ci/test/05_before_script.sh

@@ -21,9 +21,6 @@ fi
 if [ -n "$XCODE_VERSION" ] && [ -f "$OSX_SDK_PATH" ]; then
   CI_EXEC tar -C "${DEPENDS_DIR}/SDKs" -xf "$OSX_SDK_PATH"
 fi
-if [[ $HOST = *-mingw32 ]]; then
-  CI_EXEC update-alternatives --set "${HOST}-g++" \$\(which "${HOST}-g++-posix"\)
-fi
 if [ -z "$NO_DEPENDS" ]; then
   if [[ $DOCKER_NAME_TAG == *centos* ]]; then
     SHELL_OPTS="CONFIG_SHELL=/bin/dash"

contrib/containers/ci/ci.Dockerfile

@@ -33,11 +33,6 @@ RUN set -ex; \
     zip \
     && rm -rf /var/lib/apt/lists/*
 
-# Make sure std::thread and friends are available
-RUN update-alternatives --set x86_64-w64-mingw32-gcc  /usr/bin/x86_64-w64-mingw32-gcc-posix; \
-    update-alternatives --set x86_64-w64-mingw32-g++  /usr/bin/x86_64-w64-mingw32-g++-posix; \
-    exit 0
-
 # Install Clang + LLVM and set it as default
 RUN set -ex; \
     apt-get update && apt-get install ${APT_ARGS} \

doc/build-windows.md

@@ -39,8 +39,17 @@ See [dependencies.md](dependencies.md) for a complete overview.
 ## Building for 64-bit Windows
 
 The first step is to install the mingw-w64 cross-compilation tool chain:
+  - on modern systems (Ubuntu 21.04 Hirsute Hippo or newer, Debian 11 Bullseye or newer):
 
-    sudo apt-get install g++-mingw-w64-x86-64 mingw-w64-x86-64-dev
+```sh
+sudo apt install g++-mingw-w64-x86-64-posix
+```
+
+  - on older systems:
+
+```sh
+sudo apt install g++-mingw-w64-x86-64
+```
 
 Once the toolchain is installed the build steps are common: