add-on version mismatches

[Roland McGrath]

We seem to get reports fairly frequently of clueless people plopping in the
wrong linuxthreads version and trying to build.  Such people will surely
waste more of our time with their cluelessness anyway, but perhaps we could
at least save a little by having make dist write a version-stamp file into
each add-on dir and having configure barf verbosely on you if your add-on
has a stamp that's not an exact match with the main libc source.