Update Qt5 to 5.12.1

Release announcement:

https://blog.qt.io/blog/2019/02/01/qt-5-12-1-released/

Changelog:

https://wiki.qt.io/Qt_5.12.1_Change_Files

• A change was required to qt-dist.mk to always pass LOCALBASE to qmake, as Qt5 has been installed to a prefix for some time now, there should not be any harm in that, with respect to it picking up installed versions of itself during build.

PR: 235622
Exp-run by: antoine