The assumption that all configure variables had been normalized to
TRUE/<empty> turned out not to hold; convert a bit more in that
direction.
(regression from 4af38b099c741c3676aefeb20c515913aaeed666)
Change-Id: I2127c515e8a833a07c9b26ed9d693ce5a1853fe4
... in commit bf6d1f77420dcc9ece4d9f4eae1e37b427d85c6a. And it just
happens to not build with clang here.
Change-Id: Ic467abc894b9c98c81fa64cb57eafaa4cfcaa66e