Disable threads completely if OpenMP is not present.
Disable threads completely if OpenMP is not present.

This reduces the penalty paid by Mutex lock and unlock operations
in single threaded mode.

Change-Id: I185380bde73fe87e901fc434d152d6c366ff1d5d

whuaegeanse/ceres-solver,ikoruk/ceres-solver,ikoruk/ceres-solver,kartikkumar/ceres-solver,DougFirErickson/ceres-solver,kartikkumar/ceres-solver,kartikkumar/ceres-solver,entylop/ceres-solver,ysonggit/ceres-solver,timfel/ceres-bin,kartikkumar/ceres-solver,ikoruk/ceres-solver,jmwalls/ceres-solver-fork,stonier/ceres_solver,ikoruk/ceres-solver,DougFirErickson/ceres-solver,stonier/ceres_solver,jmwalls/ceres-solver-fork,stonier/ceres_solver,jmwalls/ceres-solver-fork,entylop/ceres-solver,DougFirErickson/ceres-solver,entylop/ceres-solver,whuaegeanse/ceres-solver,timfel/ceres-bin,entylop/ceres-solver,stonier/ceres_solver,whuaegeanse/ceres-solver,whuaegeanse/ceres-solver,ikoruk/ceres-solver,ysonggit/ceres-solver,DougFirErickson/ceres-solver,timfel/ceres-bin,ysonggit/ceres-solver,timfel/ceres-bin,jmwalls/ceres-solver-fork,ysonggit/ceres-solver