[Simplify redisplay-x/separate_textual_runs] Fix union build, shut up gcc

[Stephen J. Turnbull]

Olivier Galibert writes:

 > Didn't know about UNUSED syntax.  Should have looked.

Oh, yeah, you weren't around much.  Last year Jerry spent the summer
visiting someplace they were doing research on static checkers, so we
got the benefit of being the guinea pig.  Since it was Jerry's
research, he had to write the patches to make sure the checker knew
what it was doing.  Good deal, huh?

Anyway, I *think* that UNUSED (and a couple of "USED_IF_condition"
variants, see compiler.h) was the main contribution to syntax from the
static checkers.