Issue
Some of the instructions on using qmake in Windows demonstrate passing strings that contain backslashed-paths. Such usage generates a warning:

WARNING: (internal):1: Unescaped backslashes are deprecated.

Furthermore, some of the instructions also feature paths-with-spaces WITHOUT using quotes around it, leading qmake to think that the user has specified 2 paths.

I'm not sure how prevalent this issue is; I only noticed one case but it's probably good to check for others.

Example
This snippet from http://qt-project.org/doc/qt-5.0/sql-driver.html#how-to-build-the-qmysql-plugin-on-windows

qmake "INCLUDEPATH+=C:\MySQL\include" "LIBS+=C:\MYSQL\MySQL Server <version>\lib\opt\libmysql.lib" mysql.pro

Suggestion
As Volker suggested in http://qt-project.org/forums/viewthread/10225,

1. Use forward-slashes only
2. Use quotation marks whenever the path contains spaces