Mystery cracked, sorry for the noise: there is a second include directory at include/torch/csrc/api/include. We fixed it by doing:
LIBTORCH_INCLUDE_PATH = $(LIBTORCH_DIR)/include/ $(LIBTORCH_DIR)/include/torch/csrc/api/include

I will be glad to send a PR to the docs if you think it is worth talking about a non-CMake build. Thanks!