In installers/generic-linux/build-and-make-installer.sh on line 20:
git clone --recursive https://github.com/Mudlet/Mudlet.git source
no longer needs to be recursive to pull in all the submodules as that is now handled in the CMake/QMake project files which only pull in the sub-modules needed for each platform.
I believe the line should be changed to:
git clone https://github.com/Mudlet/Mudlet.git source