[tex-live] mpost assert(), please re-compile x86_64

[Karl Berry]

    currently mpost as in TeX Live trunk stops here on x86_64 GNU/Linux:

    (/usr/local/texlive/2013/texmf-dist/metapost/base/plain.mp mpost:
    ../../../texk/web2c/mplibdir/mp.w:5488: do_set_attr_head: Assertion
    `A->type==mp_structured' failed.

Doesn't happen on i386-linux, FWIW.  I can read plain.mp (and produce
output) there.  Guess we should make another build-time test for reading
plain.mp.  Sigh.

I always wonder if it's a compiler optimization bug in such cases.
Peter, any ideas, since it's your binary?

Thanks,
k