make check build failure (tst-printf-bz18872)

[Roland McGrath]

> > 2015-10-08  Roland McGrath  <roland@hack.frob.com>
> > 
> > 	[BZ #18872]
> > 	* stdio-common/Makefile ($(objpfx)tst-unbputc.out):
> > 	Move out from under [$(run-built-tests) = yes] conditional.
> 
> That looks wrong.  tst-unbputc.sh uses test_program_prefix to run a newly 
> built test.

Oh, it was unrelated.  I failed to notice that.  But why is it wrong?  I
don't see why we need to avoid defining the targets.  Adding them to
tests-special is conditionalized, so nothing will depend on that.