For building a designer plugin it is necessary to have qt being built with the qttools module. Actually this is not a valid dependency as building a designer plugin does not mean, that you are going to use the designer from this build.
My use case is being the maintainer of the Qwt library ( https://qwt.sf.net ). I never use the designer myself, but have to build its designer plugin. As Qwt supports all versions >= Qt 4.4 having to this sums up for me.