Fix location of true(1) in configure.in. This problem is masked if
makedepend (a part of imake) is installed as configure looks for it first.
If it cannot find it it just uses true(1) instead. Since upgrade of X.Org
ports to 6.8.1, makedepend is no longer necessary there at build time.
Tell it that true actually lives in /usr/bin as putting this in
CONFIGURE_ENV doesn't work.