assert.h: allow gcc to detect assert(a = 1) errors

[Jim Meyering]

Hello, I noticed that the definition of assert masks
accidental assignment errors that gcc would detect
in any other context, so wrote a patch:

  https://bugzilla.redhat.com/show_bug.cgi?id=1105335

Two questions:

  - can anyone see a problem with the patch I proposed?
  - do you think it's reasonable to push before the release?

Thanks,
Jim