qmake library path search order changed

My project has a  base library named netlib.lib.  I upgrade the project from 5.9 to 5.12，then all apps linking to netlib.lib failed, as there is a library with the same name in windows sdk (in winv6.3/um/x64).

1.  In all versions before 5.9 (including qt4),  user supplied path seemed to be searched first.

2. In 5.12,  system path is searched first. And in the generated Makefile,  netlib.lib is converted to full path (""C:\Program Files (x86)\Windows Kits\8.1\Lib\winv6.3\um\x64\netlib.lib"").

I guess this bug may be related to https://codereview.qt-project.org/#/c/212562/12.