Log Message:
Update to 1.4.21. From the changelog:

documentation:

* Consistently say "macOS" not "Mac OS X", "OS X", etc.

indexers:

* omindex:

  + Add support for gzip-compressed SVG files (.svgz).

  + Handle <title> in SVG.  Previously only <dc:title> inside \ 
<metadata> was
    considered.  If both are present, <title> now takes precedence.

testsuite:

* omegatest: Add skip-for-32-bit-time_t mechanism and use it to conditionally
  enable some testcases which fail on platforms with 32-bit time_t.

build system:

* Update to use AX_CXX_COMPILE_STDCXX which is a replacement for
  AX_CXX_COMPILE_STDCXX_11 (which we were using) which also supports newer C++
  standards versions which will be useful.  For C++11 the only difference seems
  to be that the macro now checks for attribute support - we use C++11
  attributes so that seems a good thing.

Updating during the freeze for the bug and portability fixes.