The only release-relevant thing I can see here is that it's not a release-critical bug. Thank you for bringing it to my attention so that I could downgrade it. :-) (The grave severity implies the package is unusable, which libc6-dev isn't; it doesn't seem to fit any of the criteria for serious bugs in the release policy, either.)
Well using include files belonging to another package/version is the best way to create RC bugs that you will spend weeks to find. I'm _sure_ somebody did _indeed_ run some pthread related test to check the desired behavior :-)
-- __ / ` Eric Valette /-- __ o _. 6 rue Paul Le Flem (___, / (_(_(__ 35740 Pace Tel: +33 (0)2 99 85 26 76 Fax: +33 (0)2 99 85 26 76 E-mail: eric.valette@free.fr