build, qt: Fix Windows cross-compiling with Qt 5.15

cd/05_before_script.sh

@@ -18,9 +18,7 @@ fi
 if [ -n "$XCODE_VERSION" ] && [ -f "$OSX_SDK_PATH" ]; then
   DOCKER_EXEC tar -C "${DEPENDS_DIR}/SDKs" -xf "$OSX_SDK_PATH"
 fi
-if [[ $HOST = *-mingw32 ]]; then
-  DOCKER_EXEC update-alternatives --set $HOST-g++ \$\(which $HOST-g++-posix\)
-fi
+
 if [ -z "$NO_DEPENDS" ]; then
   if [[ $DOCKER_NAME_TAG == centos* ]]; then
     # CentOS has problems building the depends if the config shell is not explicitly set

depends/packages/qt.mk

@@ -164,6 +164,7 @@ $(package)_config_opts_mingw32 += -no-dbus
 $(package)_config_opts_mingw32 += -no-freetype
 $(package)_config_opts_mingw32 += -xplatform win32-g++
 $(package)_config_opts_mingw32 += "QMAKE_CFLAGS = '$($(package)_cflags) $($(package)_cppflags)'"
+$(package)_config_opts_mingw32 += "QMAKE_CXX = '$($(package)_cxx)'"
 $(package)_config_opts_mingw32 += "QMAKE_CXXFLAGS = '$($(package)_cflags) $($(package)_cppflags)'"
 $(package)_config_opts_mingw32 += "QMAKE_LFLAGS = '$($(package)_ldflags)'"
 $(package)_config_opts_mingw32 += -device-option CROSS_COMPILE="$(host)-"

doc/build-windows.md

@@ -46,8 +46,17 @@ Acquire the source in the usual way:
 ## 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 install g++-mingw-w64-x86-64
+```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: