Windows Makefile generated with forward slash in LIBS

We configure for static builds, with a prefix, like this:
..\qtdev\qt55\configure -commercial -confirm-license -static -prefix \QtStatic\5.5 -debug-and-release -platform win32-msvc2013 -mp -opengl desktop -qt-zlib -qt-libjpeg -qt-libpng -no-openssl -no-accessibility -nomake examples -nomake tools -nomake tests -skip activeqt -skip connectivity -skip declarative -skip doc -skip enginio -skip graphicaleffects -skip location -skip macextras -skip quick1 -skip quickcontrols -skip script -skip sensors -skip svg -skip tools -skip translations -skip webkit -skip webkit-examples -skip websockets -skip winextras -skip x11extras -skip xmlpatterns -skip androidextras -skip webchannel -skip qa

Our application compiled but could not link due to many missing symbols errors. Looking at the generated Makefile, the "LIBS" property has "/QtStatic/5.5/lib" as a prefix before all of the Qt libraries. This causes the linking errors.

In earlier versions, such as Qt 5.0-5.4, the path used backslashes.