I thought I could trick arduino into using a specific version of ugfx by looking for a special include file and adding the appropriate include path for other libraries to find stuff in, but arduino adds all libraries where a specific include file is found. So looking for "ugfx-2.4.h" in my main sketch indeed just adds the folder "ugfx-2.4", but as other files are including "gfx.h" both "ugfx" and "ugfx-2.4" are added to the compiler include path because both contain "gfx.h". This might work, but will surely lead to confusion sooner or later.

So I'll forget about that for now.