* bpo-30947, bpo-31170: Update expat from 2.2.1 to 2.2.4

* Upgrade libexpat embedded copy from version 2.2.1 to 2.2.3 to get security
  fixes.

* Update libexpat from 2.2.3 to 2.2.4. Fix copying of partial
  characters for UTF-8 input (libexpat bug 115):
  https://github.com/libexpat/libexpat/issues/115

* Define XML_POOR_ENTROPY when compiling expat

Issue #21668: Link audioop, _datetime, _ctypes_test modules to libm, except on
Mac OS X. Patch written by Xavier de Gaye.

As of Xcode 7, SDKs for Apple platforms now include textual-format stub
libraries whose file names have a .tbd extension rather than the
standard OS X .dylib extension.  The Apple compiler tool chain handles
these stub libraries transparently and the installed system shared libraries
are still .dylibs.  However, the new stub libraries cause problems for
third-party programs that support building with Apple SDKs and make
build-time decisions based on the presence or paths of system-supplied
shared libraries in the SDK.  In particular, building Python itself with
an SDK fails to find system-supplied libraries during setup.py's build of
standard library extension modules.  The solution is to have
find_library_file() in Distutils search for .tbd files, along with
the existing types (.a, .so, and .dylib).  Patch by Tim Smith.

Known limitations of the current implementation:

- documentation changes are incomplete
- there's a reference leak I haven't tracked down yet

The leak is most visible by running:

  ./python -m test -R3:3 test_importlib

However, you can also see it by running:

  ./python -X showrefcount

Importing the array or _testmultiphase modules, and
then deleting them from both sys.modules and the local
namespace shows significant increases in the total
number of active references each cycle. By contrast,
with _testcapi (which continues to use single-phase
initialisation) the global refcounts stabilise after
a couple of cycles.