1) Prepare libmpdec for the 2.4.0 release.  None of the following changes affects
   _decimal:

      o Make all "mpd_t to C integer" conversion functions available in both the
        64-bit and the 32-bit versions.

      o Make all mixed mpd_t/C integer arithmetic functions available in the
        32-bit version.

      o Better handling of __STDC_LIMIT_MACROS for C++ users.

      o Add struct tags (at the request of C++ users).

2) Check for libmpdec.so.2 if --with-system-libmpdec is used.
Document that @property can incorporate a docstring from the getter method.  Improve readabilty with additional whitespace.
#19620: Fix typo in docstring (noticed by Christopher Welborn).
Fix broken link in html.entities docs.