bpo-36465: Make release and debug ABI compatible

[vstinner]

Release build and debug build are now ABI compatible: the Py_DEBUG
define no longer implies Py_TRACE_REFS define which introduces the
only ABI incompatibility.

A new "./configure --with-trace-refs" build option is now required to
get Py_TRACE_REFS define which adds sys.getobjects() function and
PYTHONDUMPREFS environment variable.

Changes:

• Add ./configure --with-trace-refs
• Py_DEBUG no longer implies Py_TRACE_REFS

https://bugs.python.org/issue36465

[vstinner]

I rebased my PR and I added a NEWS entry.

[vstinner]

I reverted the following change:

* The "d" flag of SOABI (sys.implementation.cache_tag) is now
  only added by --with-trace-refs. It is no longer added by
  --with-pydebug.

I found an issue between libpython and SOABI: https://mail.python.org/pipermail/python-dev/2019-April/157186.html

I will work on a separated PR to remove "d" from SOABI, to handle properly the libpython issue.

This PR is not restricted to "Py_DEBUG no longer implies Py_TRACE_REFS".

[vstinner]

@pitrou: You approved the idea on the python-dev, would you mind to approve the PR as well?
https://mail.python.org/pipermail/python-dev/2019-April/157185.html

@gvanrossum: Ditto. You approved the idea on the python-dev, would you mind to approve the PR as well?
https://mail.python.org/pipermail/python-dev/2019-April/157066.html

[gvanrossum]

I'm sorry, I'm not the one who should review this. I have no relevant knowledge any more.

[vstinner]

@pablogsal: Thanks, I fixed the 3 typos.