[PATCH] PR69699 document why __GLIBCXX__ macro is useless

[Jonathan Wakely <jwakely at redhat dot com>]

The closest thing we have to a version macro in libstdc++ is
__GLIBCXX__ which holds the valid of gcc/DATESTAMP from the source
tree. That's useless for version checking or feature testing because
snapshots have arbitrary values and there's no total order across
branches (a later date does not mean a "better" release with more
features implemented). This updates the docs to point out it isn't
very useful. I've also removed the list of release dates, linking to
the online release timeline instead, so we don't have to keep adding
to the list. This means the information is not included in the sources
and you need to be online to find a date, but since the dates are not
very useful anyway I don't think this is a problem.

	PR libstdc++/69699
	* doc/xml/manual/abi.xml (abi.versioning.history): Explain why the
	_GLIBCXX__ macro is not useful. Remove redundant date information
	and link to the GCC release timeline.
	(abi.versioning.active): Move partial sentence into the previous
	paragraph.
	* doc/html/*: Regenerate.

Committed to trunk.

Attachment: patch.txt
Description: Text document