[Python] Remove ARROW_BUILD_DIR in building pyarrow C++

[asfimport]

When building pyarrow, I see the following warning:

CMake Warning:
  Manually-specified variables were not used by the project:

    ARROW_BUILD_DIR

While we have a note in our docs (https://arrow.apache.org/docs/dev/developers/python.html#build-and-test) that says:

If you used a different directory name for building Arrow C++ (by default it is named “build”), then you should also set the environment variable ARROW_BUILD_DIR='name_of_build_dir'. This way PyArrow can find the Arrow C++ built files.

I see in the setup.py code that we check for this env variable and pass it to CMake, but it's not actually used in any of the CMakeLists.txt files for pyarrow.

This might have been accidentally changed in one of the recent cmake refactors? (cc @kou @AlenkaF)

Reporter: Joris Van den Bossche / @jorisvandenbossche
Assignee: Alenka Frim / @AlenkaF

PRs and other links:

• GitHub Pull Request #14498

_{Note: This issue was originally created as ARROW-18126. Please see the migration documentation for further details.}

[asfimport]

Alenka Frim / @AlenkaF:
This variable was added in the commit of the PyArrow refactoring:

ea45ed4

and most of it was included in the test section which is now removed. The rest of it was in the flight section which is now changed but I haven't been able to found a commit at this moment.

In any case, I think this variable is not needed anymore and can be removed. Great catch Joris!

[asfimport]

Alenka Frim / @AlenkaF:
Ok, this is the commit that removed the code from the flight setup in PyArrow C++ CMake file 0eaf8e2 that used ARROW_BUILD_DIR.

It is completely safe and sensible to remove the env variable as it is not needed anymore - will make a PR.

[asfimport]

Kouhei Sutou / @kou:
Issue resolved by pull request 14498
#14498