Add options to parallelize builds.
Add options to parallelize builds.

AppVeyor build servers have two cores, so why not use them?

google/googletest,gianricardo/googletest,jlanecox/googletest,google/googletest,antidotcb/googletest,jlanecox/googletest,fuchsia-mirror/third_party-googletest,gianricardo/googletest,xnagireddy/googletest,jlanecox/googletest,google/googletest,xnagireddy/googletest,empiredan/googletest,empiredan/googletest,fuchsia-mirror/third_party-googletest,antidotcb/googletest,jlanecox/googletest,empiredan/googletest,gianricardo/googletest,antidotcb/googletest,xnagireddy/googletest,antidotcb/googletest,xnagireddy/googletest,fuchsia-mirror/third_party-googletest,google/googletest,fuchsia-mirror/third_party-googletest