BUG: add linux guard per #21386

[jmgurney]

Closes #21386.

[jmgurney]

This compiles fine on FreeBSD:

$ pip install -e git+https://github.com/jmgurney/numpy.git'#egg=numpy'
Obtaining numpy from git+https://github.com/jmgurney/numpy.git#egg=numpy
  Cloning https://github.com/jmgurney/numpy.git to ./p/src/numpy
  Running command git clone --filter=blob:none --quiet https://github.com/jmgurney/numpy.git /home/freebsd/haws/p/src/numpy
load: 0.33  cmd: git 72349 [running] 14.71r 0.93u 1.14s 21% 28764k
  Resolved https://github.com/jmgurney/numpy.git to commit a8663f6c368d4b20e20ea7b6612aced63d334a4f
  Running command git submodule update --init --recursive -q
  Installing build dependencies ... done
  Checking if build backend supports build_editable ... done
  Getting requirements to build wheel ... done
  Preparing metadata (pyproject.toml) ... done
Installing collected packages: numpy
  Attempting uninstall: numpy
    Found existing installation: numpy 1.21.0.dev0+436.g5c7002926
    Uninstalling numpy-1.21.0.dev0+436.g5c7002926:
      Successfully uninstalled numpy-1.21.0.dev0+436.g5c7002926
  Running setup.py develop for numpy
ERROR: pip's dependency resolver does not currently take into account all the packages that are installed. This behaviour is the source of the following dependency conflicts.
[...]
Successfully installed numpy-1.23.0.dev0+1084.ga8663f6c3

Note that the hashes differ, only because I was told too late that commit messages have to have a specific format to them, and had to amend the commit.

[charris]

Thanks @jmgurney .

[InessaPawson]

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