devel/py-cffi: Backport upstream pull request #56

Backport upstream pull request #56 [1] which fixes (double) building for
Python packackges that use py-cffi to compile their modules.

This issue manifested itself in the 'install' target (stage) of Python
port building, causing the module that was already built at the 'build'
stage to be built again, except *without* the build arguments provided
in the build stage.

These symptoms were observable in security/py-cryptography [2]

While I'm here:

• Add regression-test target and TEST_DEPENDS

unit tests: 1 failed, 1033 passed, 80 skipped in 70.56 seconds

Note: Failure in test_array_type already exists in current version, and

has been reported upstream [3]

[1] https://bitbucket.org/cffi/cffi/pull-request/56/
[2] https://github.com/pyca/cryptography/pull/1635
[3] https://bitbucket.org/cffi/cffi/issue/178/

Approved by: wg (maintainer)