Some of the instructions on using qmake in Windows demonstrate passing strings that contain backslashed-paths. Such usage generates a warning:
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.
This snippet from http://qt-project.org/doc/qt-5.0/sql-driver.html#how-to-build-the-qmysql-plugin-on-windows
As Volker suggested in http://qt-project.org/forums/viewthread/10225,
- Use forward-slashes only
- Use quotation marks whenever the path contains spaces