Force use of single-thread in Problem::Evaluate() without OpenMP.
Force use of single-thread in Problem::Evaluate() without OpenMP.

- Ensure that, as in the preprocessor, we force num_threads to be 1
  if Ceres was compiled without OpenMP to prevent a CHECK() failure
  in internal code.

Change-Id: I49d7bdeff81e5bb24aaaa3945018d9d012cd9df9

whuaegeanse/ceres-solver,whuaegeanse/ceres-solver,ysonggit/ceres-solver,ysonggit/ceres-solver,whuaegeanse/ceres-solver,whuaegeanse/ceres-solver,ysonggit/ceres-solver,DougFirErickson/ceres-solver,DougFirErickson/ceres-solver,DougFirErickson/ceres-solver,ysonggit/ceres-solver,DougFirErickson/ceres-solver