Remove duplicated C compiler option from tests that is already set by the test runner via the cpp11 tag.

See #2509.

Use the print function in the nonecheck example

Enable some tests that were unintentionally not being executed because docstrings in cdef functions are not visible to Python.

Repair incorrect return value usage in test runner when checking gcc version. Fails on Windows when gcc is not available (with an error trying to regex parse None).

Make a test work in Py3.7 that changed behaviour in Python 3.7 ("generator_stop" future import became active by default).

Closes #2494.