Move QMAKE_LIBS_QT_ENTRY adjustment to bitcoin side of build

It could just as well be on either part of the gitian build, but to safely put it on the Qt side would require bumping the filename, and every gitian user rebuilding it.
v0.5.3.1 put it on the Bitcoin side, and this is easier to work with and keep safe, so I'm moving it.

Use qmake MINGW_THREAD_BUGFIX=0 to disable