Checking for the exact match of __GXX_ABI_VERSION created more problems
(including for both Fedora and Debian packagers, see
https://github.com/wxWidgets/wxWidgets/pull/828) than it solved
(approximately 0), so relax it and assume that future g++ versions will
remain broadly compatible with the current ABI, which seems like a safe
assumption so far.
It's not really clear if there is any value in having this ABI check at
all, or if we should remove CheckBuildOptions() and all the code calling
it entirely, as it seems that there is no way to trigger this check
during run-time without getting a link error first. But keep it for now,
just because it's simpler to keep it than to remove it.
7f1d08d5bd