make builds fail to land in $$BUILD_TMPDIR$$

[fmeum]

I am not yet sure why, but some of my make builds of third-party libraries fail due to the use of $$EXT_BUILD_ROOT$$/{root} instead of $$BUILD_TMPDIR$$ in:
https://github.com/bazelbuild/rules_foreign_cc/blob/a7105dafb067c785cc9ade2c70b0d0ce967490fc/foreign_cc/make.bzl#L59
All of my builds pass with $$BUILD_TMPDIR$$ in that spot, which is also the value used by configure_make.
Edit: There are some issues with dangling symlinks with that change in other builds.

@UebelAndre Could you briefly explain why some rules use $$EXT_BUILD_ROOT$$/{root} and others use $$BUILD_TMPDIR$$? If I understand this a bit better, I could probably start working on a PR that fixes the build issues I am seeing without introducing new ones.