qmake: could not find a Qt installation of ''

For others in my situation, the solution was:

qmake -qt=qt5

This was on Ubuntu 14.04 after install qt5-qmake. qmake was a symlink to qtchooser which takes the -qt argument.

As Debian Qt's maintainer please allow me to suggest you to not use qtx-default. Please read qtchooser's man page, the solution is described there. If you are interested in packaging an app you can also take a look at this blog post I made explaining how to do it

# method 1
QT_SELECT=qt5 qmake

# method 2:
export QT_SELECT=qt5
qmake
... more qt commands here

# method 3:
make -qt5

To use Qt 4, just replace the qt5 with qt4

Update 20210202: starting from Debian 11 (bullseye) the packages qtx-default do not longer exist. Same goes for Ubuntu, but I don't know in which specific version. If you know of a package that still has the dependency (mostly non-Debian official packages) please file a bug. Same goes for Wiki pages, etc.

You could check path to qmake using which qmake.

Consider install qt4-default or qt5-default depends what version of qt you want use.

You could also use qtchooser - a wrapper used to select between Qt development binary versions.

sudo apt-get install qt5-default works for me.

$ aptitude show qt5-default

tells that

This package sets Qt 5 to be the default Qt version to be used when using development binaries like qmake. It provides a default configuration for qtchooser, but does not prevent alternative Qt installations from being used.