“libcuda.so not found” when trying to `make` matrixMulDrv cuda sample after apt install nvidia-cuda-toolkit

I installed cuda using sudo apt install nvidia-cuda-toolkit (not using the instructions on the Nvidia site). This is known to not install the samples, so I went on github to download them, as explained in another question.

I cd'ed into Samples/deviceQuery and tried make, but it complained that nvcc was not found, so I looked at the Makefile and changed the command to CUDA_PATH=/usr make and it worked. This works because the Makefile has the definition CUDA_PATH ?= /usr/local/cuda, while apt install nvidia-cuda-toolkit actually puts nvcc at /usr/bin/nvcc.

Then I tried doing the same thing in Samples/matrixMulDrv, but it gave me a different error:

>>> WARNING - libcuda.so not found, CUDA Driver is not installed.  Please re-install the driver. <<<

Doing locate libcuda.so says I do have it, and it's at /usr/lib/x86_64-linux-gnu.

How can I make matrixMulDrv?