Makefiles use wrong Python when building on Linux for Windows

[gilles-peskine-arm]

In the GNU makefiles, WINDOWS is defined when building on Windows, and WINDOWS_BUILD is defined when building for Windows. The selection of Python (python on Windows, python3 elsewhere — a practical choice that works for us) is based on WINDOWS_BUILD but should be based on WINDOWS.

This does not currently cause any CI failure or uncaught failure, but it's very fragile.

This causes an error when building with mingw on Linux, as in tests/scripts/all.sh build_mingw. Since PYTHON is python, which is Python 2.7, all Python code invoked from makefiles runs with Python 2. For generate_test_code.py, this still works because this script was last modified a long time ago when we ensured that our scripts worked with both Python 2.x and Python 3.x. In the development branch, tests/Makefile runs $(PYTHON) scripts/generate_psa_tests.py --list to enumerate potentially generated test data files, which results in the error

  File "scripts/generate_psa_tests.py", line 40
    def psa_want_symbol(name: str) -> str:
                            ^
SyntaxError: invalid syntax

This is currently harmless on the CI because the error occurs in a $(shell …) make function which does not check the command's return status, and all the generated test data is pre-generated before doing the build.