Repoint the last TODO.md citations at the tracker

docs/building.md sent a reader to TODO.md for what a stdlib replacement must
provide; the AKERR_USE_STDLIB=OFF build does not compile at all, which is issue
#12, so it says that instead.

Found while sweeping the consumer repositories: the handler macros trip
-Wimplicit-fallthrough, which is why libakgl cannot adopt -Wextra. That was
recorded only in libakgl and is now issue #18 -- the fourth defect in this
library found written down in a consumer rather than here.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Co-Authored-By: Andrew Kesterson <andrew@aklabs.net>

docs/building.md

@@ -58,4 +58,6 @@ cmake --install build
 `PATH_MAX` and `NULL` are used unconditionally but only included under the
 stdlib branch, so the header's includes need untangling before
 `-DAKERR_USE_STDLIB=OFF` builds. The list above still states what a replacement
-must provide. See [TODO.md](../TODO.md).
+must provide. That configuration does not currently compile -- `bool`, `PATH_MAX` and `NULL`
+are used unconditionally but included only under the stdlib branch. See
+[issue #12](https://source.starfort.tech/andrew/libakerror/issues/12).