message: build, qt: Specify QMAKE_CXX explicitly
  This change allows to drop the `update-alternatives` step if the
  `g++-mingw-w64-x86-64` package has been installed.
diff: @@ -163,6 +163,7 @@ $(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)-"
  