make deb is broken

[Bohdan Horbeshko]

CMake Error at CMakeLists.txt:318 (add_subdirectory):
  add_subdirectory given source "third-party/imgui" which is not an existing
  directory.


CMake Error at CMakeLists.txt:319 (add_subdirectory):
  add_subdirectory given source "third-party/lz4" which is not an existing
  directory.

Though both directories are actually present and non-empty.