Qtcreator 3.0 font issue under Ubuntu: ugly font with small size

[Jctual]

I use a fresh new Lubuntu 14.04.
I compile latest Qtlibrary and latest qtcreator 3.0 from the source.

The fonts of Qtcreator (title, task bar, ..) are in very small size and in an ugly serif font family.
If associating a stylesheet when launching qtcreator, it is posible to change the size, but the font family is never changed.

When installing qtcreator directly with the online installer, it works fine: the system font Ubuntu is used and the size is correct. Everything is perfectly readable.

Is it possible to configure Qtcreator when compiling from the source to get the same behavior as with the online installer ?

Any hint will be appreciated.