Could you add examples how to add the path to CMAKE_PREFIX_PATH?

There are 2 ways to add:

1. cmake ... -DCMAKE_PREFIX_PATH=...: CMake variable: https://cmake.org/cmake/help/latest/variable/CMAKE_PREFIX_PATH.html
2. export CMAKE_PREFIX_PATH=...: Environment variable: https://cmake.org/cmake/help/latest/envvar/CMAKE_PREFIX_PATH.html

I think the latter is better for most cases because the CMAKE_PREFIX_PATH environment variable may be defined in other place.

If we specify cmake -DCMAKE_PREFIX_PATH=${ARROW_ROOT}, the pre-defined CMAKE_PREFIX_PATH environment variable value is ignored. (I didn't confirm it yet but I think so. Sorry.) We can use the pre-defined CMAKE_PREFIX_PATH environment variable value by cmake -DCMAKE_PREFIX_PATH=${ARROW_ROOT}${CMAKE_PREFIX_PATH:+:${CMAKE_PREFIX_PATH}. (${X:+...} is a variable substitution syntax. See also ${parameter:+word} in https://www.gnu.org/software/bash/manual/html_node/Shell-Parameter-Expansion.html . This is used here to not include an empty path such as cmake -DCMAKE_PREFIX_PATH=${ARROW_ROOT}:.)

We can use the variable substitution syntax with export CMAKE_PREFIX_PATH=... case:

export CMAKE_PREFIX_PATH=${ARROW_ROOT}${CMAKE_PREFIX_PATH:+:${CMAKE_PREFIX_PATH}}

Thanks a lot! I've added a couple of examples, let me know if it makes sense or you think there's something more missing.