generate_type_definitions maintained a list of ctuples for which code
was already generated, but the logic did not match the logic used in
generate_type_header_code, which does not generate code if the entry is
marked as unused.

If the first ctuple of a certain type was marked unused, but later
instances were not, this could result in no code being generated for
the ctuple at all. Resolve by verifying whether the ctuple entry is used
before adding it to the list.
Co-authored-by: Kirk Meyer <kirk.meyer@ravenind.com>

* Updated URL for the "embedding Python" link in external_C_code.rst
  in the userguide.
* The freenet.org.nz domain has clearly expired at some point and now
  the original link goes spam links, but there are copies in the
  Wayback Machine.
* The updated (fixed) URL uses the most recent archive for the
  original page (author: June, 2004; web archive: Feb, 2012).

Follows Python behaviour, but excludes "__pyx_…" names in utility code.

Closes GH-1382.

Currently excludes PyPy2.
Closes GH-2781.

Add "tp_pypy_flags" field to PyTypeObject struct when compiling in PyPy 6.0+ in order to avoid C compiler warnings about an uninitialised struct field.

Right now it's used in a few simple cases (function call and single assignment)
Don't attempt to remove reference types.
Be wary of the "safety check" for double moves when things like result() are called twice
Made sure fallback option was genuinely c++03 complient so that test should pass on clang

This reverts commit 52635828.

multiprocessing requires some additional setup on Windows, which I don't currently want to implement in setup.py.

Fixes GH-3534.

Fixes GH-3534.

Suppress non-error output of the C compiler in test runner unless there is a test failure or at least some error output as well.

Fixes #3531.

Fixes #3531.

Closes #2753.