- Previously we had separate variables for each of the threading
backends, each of which were made mutually exclusive via
cmake_dependent_option(). This has unfortunate side-effects when
trying to disable options if they are not currently enabled, in which
case they are not defined.
- As all the threading options are mutually exclusive, this replaces
all threading option variables with a single variable: CERES_THREADS,
which is constrained to take the value of only the available threading
backends.
Change-Id: I0822eefbac9a30772907b7732add365b37cc8ca0
Alex Stewart