expat: update to 2.3.0.

Release 2.3.0 Thu March 25 2021
        Bug fixes:
            #438  When calling XML_ParseBuffer without a prior successful call to
                    XML_GetBuffer as a user, no longer trigger undefined behavior
                    (by adding an integer to a NULL pointer) but rather return
                    XML_STATUS_ERROR and set the error code to (new) code
                    XML_ERROR_NO_BUFFER. Found by UBSan (UndefinedBehaviorSanitizer)
                    of Clang 11 (but not Clang 9).
            #444  xmlwf: Exit status 2 was used for both:
                    - malformed input files (documented) and
                    - invalid command-line arguments (undocumented).
                    The case of invalid command-line arguments now
                    has its own exit status 4, resolving the ambiguity.

        Other changes:
            #439  xmlwf: Add argument -k to allow continuing after
                    non-fatal errors
            #439  xmlwf: Add section about exit status to the -h help output
  #422 #426 #447  Windows: Drop support for Visual Studio <=14.0/2015
            #434  Windows: CMake: Detect unsupported Visual Studio at
                    configure time (rather than at compile time)
      #382 #428  testrunner: Make verbose mode (argument "-v") report
                    about passed tests, and make default mode report about
                    failures, as well.
            #442  CMake: Call "enable_language(CXX)" prior to tinkering
                    with CMAKE_CXX_* variables
            #448  Document use of libexpat from a CMake-based project
            #451  Autotools: Install CMake files as generated by CMake 3.19.6
                    so that users with "find_package(expat [..] CONFIG [..])"
                    are served on distributions that are *not* using the CMake
                    build system inside for libexpat packaging
      #436 #437  Autotools: Drop obsolescent macro AC_HEADER_STDC
      #450 #452  Autotools: Resolve use of obsolete macro AC_CONFIG_HEADER
            #441  Address compiler warnings
            #443  Version info bumped from 7:12:6 to 8:0:7
                    due to addition of error code XML_ERROR_NO_BUFFER
                    (see https://verbump.de/ for what these numbers do)

        Infrastructure:
      #435 #446  Replace Travis CI by GitHub Actions

        Special thanks to:
            Alexander Richardson
            Oleksandr Popovych
            Thomas Beutlich
            Tim Bray
                and
            Clang LeakSan, Clang 11 UBSan and the Clang team

(wiz)