Merge bitcoin#29747: depends: fix mingw-w64 Qt DEBUG=1 build

b7e7e72 depends: fix mingw-w64 Qt DEBUG=1 build (fanquake)

Pull request description:

  The issue is that compilation is done with `x86_64-w64-mingw32-g++-posix`, but then linking is done with `x86_64-w64-mingw32-g++`.

  I'm guessing this has been broken since bitcoin#24131 (01d1845), but have not checked.

  Fixes bitcoin#29734.
  Unblocks bitcoin#29527 (`DEBUG=1` builds can be tested).

ACKs for top commit:
  hebasto:
    ACK b7e7e72, tested on Ubuntu 22.04 with the [installed](bitcoin#29734 (comment)) `g++-mingw-w64-x86-64` package.
  TheCharlatan:
    ACK b7e7e72

Tree-SHA512: 9e24e84046c0489c20971bb9c68d1a643c233837193c184f61bff79dfc8d7397a5c5526ac1a205ad423920f2589559cd01cb104ceb7f89515bb6421510d82ca9

Author: fanquake

depends/packages/qt.mk

@@ -177,6 +177,7 @@ $(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)_cxxflags) $($(package)_cppflags)'"
+$(package)_config_opts_mingw32 += "QMAKE_LINK = '$($(package)_cxx)'"
 $(package)_config_opts_mingw32 += "QMAKE_LFLAGS = '$($(package)_ldflags)'"
 $(package)_config_opts_mingw32 += "QMAKE_LIB = '$($(package)_ar) rc'"
 $(package)_config_opts_mingw32 += -device-option CROSS_COMPILE="$(host)-"