test-stddef: test for (some) offsetof bugs See the mailing list for a more comprehensive patch that works around the Solaris bug. * tests/test-stddef.c: Enhance test to ensure correct type of offsetof. * doc/posix-headers/stddef.texi (stddef.h): Document a Solaris bug that we are not fixing at this time. Signed-off-by: Eric Blake <eblake@redhat.com>