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.

Closes #2484.

Fix calling va_arg function with a NULL argument from generated C++ code

C++ allows NULL to be a literal 0 [1], and this actually happens on some Linux
systems when linux/stddef.h happens to be included [2]. When 0 is passed to a
variadic function as a 7th or later argument, it is passed on the stack, and
Clang encodes this on AMD64 with "mov dword ptr [rsp], 0" because it is an
int. This sets lower 32 bits to zero, but leaves upper 32 bits unchanged.
When they happen to be non zero, the called function that expects the last
argument to be a zero pointer reads past the last intended argument and
eventually segfaults.

[1] https://en.cppreference.com/w/cpp/types/NULL
[2] https://stackoverflow.com/a/31285400/1687334
[3] https://godbolt.org/g/o4Av7Q