Qt4, Qt5: create symlinks $(TARGET)-qmake-qtX in $(PREFIX)/bin

That way a configure script looking for "qmake-qt4" or "qmake-qt5"
with AC_PATH_PROG will find e.g. x86_64-w64-mingw32-qmake-qt5
properly. See
http://thread.gmane.org/gmane.comp.gnu.mingw.cross-env/3424/focus=3426