VST3 build target just won't build

I am trying to build a VST3 plugin from my JUCE project.
I have set the path to the VST3 SDK accordingly under File->Preferences.
When I check “Build VST” and run make, a .so file is generated as expected.
Also checking “Build VST3” seems to make no difference at all - all that is built is the VST2 target (.so file).
When I uncheck “Build VST” and leave “Build VST3” checked, running make produces:

$ make
make: Nothing to be done for ‘all’.

Strangely enough, building the VST2 target even works when I enter an erroneous VST path under File->Preferences.

How can I build a VST3 plugin?

Build configuration is Linux Makefile.

We don’t use the VST2 part of the SDK - that bit is all handled by custom JUCE code.

Linux VST3 support is not currently available, but it is on our roadmap: