It turns out the answer was not specify a path for the different variables, as was suggested in the posts I had found during my search. The answer (as mentioned in this post linked by @RichardBarber) was simply to specify the compiler's name in the command:

cmake -B build -DCMAKE_BUILD_TYPE=Release -DCC="clang" -DCXX="clang++" -DOpenMP_C_FLAGS=-fopenmp=lomp -DOpenMP_C_LIB_NAMES="libomp"

There was no need to assign the paths to any binaries or libraries.