Fix shutting down FastClock's thread on OSX.
Fix shutting down FastClock's thread on OSX.

nickhutchinson/benchmark,xlqian/benchmark,codygray/benchmark,nickhutchinson/benchmark,Photonomie/benchmark,LepelTsmok/benchmark,eliben/benchmark,subailong/benchmark,fuchsia-mirror/third_party-benchmark,chandlerc/benchmark,everbase/benchmark,everbase/benchmark,fuchsia-mirror/third_party-benchmark,mockingbirdnest/benchmark,google/benchmark,eliben/benchmark,efcs/benchmark,mockingbirdnest/benchmark,subailong/benchmark,ntx-/benchmark,mockingbirdnest/benchmark,ntx-/benchmark,xlqian/benchmark,quantumsteve/benchmark,nickhutchinson/benchmark-cxx03,fuchsia-mirror/third_party-benchmark,Photonomie/benchmark,gezidan/benchmark,eliben/benchmark,chemhack/benchmark,dcoeurjo/benchmark,nickhutchinson/benchmark,Photonomie/benchmark,bowlofstew/benchmark,fuchsia-mirror/third_party-benchmark,codygray/benchmark,Photonomie/benchmark,xlqian/benchmark,chemhack/benchmark,bowlofstew/benchmark,LepelTsmok/benchmark,chandlerc/benchmark,everbase/benchmark,ntx-/benchmark,disconnect3d/benchmark,chemhack/benchmark,quantumsteve/benchmark,nickhutchinson/benchmark-cxx03,chandlerc/benchmark,google/benchmark,nickhutchinson/benchmark-cxx03,efcs/benchmark,disconnect3d/benchmark,dcoeurjo/benchmark,google/benchmark,chemhack/benchmark,quantumsteve/benchmark,bowlofstew/benchmark,mockingbirdnest/benchmark,efcs/benchmark,subailong/benchmark,efcs/benchmark,ntx-/benchmark,nickhutchinson/benchmark,bowlofstew/benchmark,gezidan/benchmark,disconnect3d/benchmark,subailong/benchmark,gezidan/benchmark,eliben/benchmark,xlqian/benchmark,gezidan/benchmark,everbase/benchmark,LepelTsmok/benchmark,codygray/benchmark,dcoeurjo/benchmark,codygray/benchmark,nickhutchinson/benchmark-cxx03,quantumsteve/benchmark,disconnect3d/benchmark,LepelTsmok/benchmark,chandlerc/benchmark