Pass CFLAGS and LDFLAGS when running tests

[ob-stripe]

r? @brandur-stripe
cc @stripe/api-libraries

Use the LDFLAGS and CFLAGS environment variables when running tests.

Context for why this is useful:

• with Python 2, we install the requests[security] meta-package, which contains the cryptography package:

  stripe-python/setup.py

  Line 54 in c5b9414

  'requests[security] >= 2.20; python_version < "3.0"',

• PyPy 2 cannot use the prebuilt wheel and has to compile from source
• In order for the compilation to succeed on macOS, you need to pass the right LDFLAGS and CFLAGS value: https://cryptography.io/en/latest/installation/#building-cryptography-on-macos

With this PR, I can run this command on my laptop:

$ LDFLAGS="-L$(brew --prefix openssl@1.1)/lib" \
    CFLAGS="-I$(brew --prefix openssl@1.1)/include" \
    make test

and verify that tests pass with PyPy 2.

[brandur-stripe]

Nice.

Would you mind just putting a brief comment on that line saying that it's there to enable successful crypto builds on Mac OS? Otherwise, LGTM.

ptal @ob-stripe