Adding tests for the code snippets in "Calling C functions".

Adding tests for the code snippets in docs/src/quickstart/cythonize.rst

The code snippets from external.rst are now tested with the mode "compile". Fixed a small bug with a cdef that was forgotten.

Moved the code snippets from the .rst file to the examples directory. Those code snippets are now tested with the mode "compile".

Enable tests for all the files in the example directory.

This closes #2323.

Compile time env available from cython command line options

Formatted runtests.py to make it respect the pep8 conventions of try...except.

Removed an unused variable.

Removed the unused variable "WORKDIR" from runtests.py.

See #2319.

Fix purepython builtin type strings

close #2314

Speed up cpdef method override checks also for Python subclasses with slots by assuming that slot attributes themselves never override type fields in the parent class (Python raises an error if you try).
See #2313.

This does not help for classes with "__slots__", nor does it help for alternating calls to different objects.
There is probably still space for improvements, e.g. by using an array of object dict versions.
See #2313.

Using __new__ was made an error 8 years ago, keeping this in the docs just causes confusion at this point (e.g. #2311).

Make raise-statements inside of nogil blocks automatically acquire the GIL, instead of requiring an explicit ``with gil`` block around them.

Fix: reallowing tp_clear() in a subtype of an @no_gc_clear extension type generated an invalid C function call to the (non-existent) base type implementation.
Closes #2309.

Add decrlarations from ceval.h

Add PyEval_InitThreads and PyEval_ThreadsInitialized