Stop linking pthread.
Stop linking pthread.

It was only needed by gtest, which is no longer used.
(Intesting note: I checked when the -lpthread flag was added, and it's
been around since the first revision of build.ninja, which used to be
checked in before configure.py existed. Back then, it looks like '@' was
used to dereference built-in variables, and build outputs were also
prefixed by '@'!).

dendy/ninja,nicolasdespres/ninja,nafest/ninja,ThiagoGarciaAlves/ninja,atetubou/ninja,ndsol/subninja,fuchsia-mirror/third_party-ninja,synaptek/ninja,Qix-/ninja,Maratyszcza/ninja-pypi,ilor/ninja,Qix-/ninja,Ju2ender/ninja,synaptek/ninja,martine/ninja,Ju2ender/ninja,martine/ninja,dorgonman/ninja,ilor/ninja,mohamed/ninja,nicolasdespres/ninja,AoD314/ninja,iwadon/ninja,jendrikillner/ninja,nafest/ninja,ctiller/ninja,kissthink/ninja,vvvrrooomm/ninja,rjogrady/ninja,moroten/ninja,martine/ninja,bradking/ninja,ilor/ninja,jimon/ninja,jendrikillner/ninja,bmeurer/ninja,metti/ninja,ctiller/ninja,colincross/ninja,hnney/ninja,fuchsia-mirror/third_party-ninja,Qix-/ninja,maruel/ninja,ninja-build/ninja,Ju2ender/ninja,atetubou/ninja,liukd/ninja,ignatenkobrain/ninja,mgaunard/ninja,iwadon/ninja,nicolasdespres/ninja,jimon/ninja,moroten/ninja,bmeurer/ninja,metti/ninja,maruel/ninja,juntalis/ninja,moroten/ninja,sgraham/ninja,lizh06/ninja,mgaunard/ninja,sxlin/dist_ninja,metti/ninja,sgraham/ninja,metti/ninja,mohamed/ninja,bradking/ninja,tfarina/ninja,bmeurer/ninja,mgaunard/ninja,kissthink/ninja,automeka/ninja,ndsol/subninja,ninja-build/ninja,tfarina/ninja,ignatenkobrain/ninja,liukd/ninja,ctiller/ninja,dorgonman/ninja,sxlin/dist_ninja,ndsol/subninja,rjogrady/ninja,Maratyszcza/ninja-pypi,rjogrady/ninja,liukd/ninja,Qix-/ninja,juntalis/ninja,sxlin/dist_ninja,nafest/ninja,AoD314/ninja,rjogrady/ninja,ThiagoGarciaAlves/ninja,bradking/ninja,mydongistiny/ninja,mydongistiny/ninja,AoD314/ninja,maruel/ninja,AoD314/ninja,liukd/ninja,iwadon/ninja,juntalis/ninja,lizh06/ninja,dendy/ninja,ignatenkobrain/ninja,atetubou/ninja,fuchsia-mirror/third_party-ninja,kissthink/ninja,ignatenkobrain/ninja,nafest/ninja,autopulated/ninja,juntalis/ninja,martine/ninja,colincross/ninja,synaptek/ninja,hnney/ninja,vvvrrooomm/ninja,maruel/ninja,jimon/ninja,sxlin/dist_ninja,sgraham/ninja,Maratyszcza/ninja-pypi,tfarina/ninja,mohamed/ninja,hnney/ninja,jimon/ninja,mydongistiny/ninja,ndsol/subninja,dorgonman/ninja,nico/ninja,kissthink/ninja,bmeurer/ninja,nico/ninja,vvvrrooomm/ninja,dorgonman/ninja,dendy/ninja,nico/ninja,bradking/ninja,ninja-build/ninja,automeka/ninja,iwadon/ninja,ThiagoGarciaAlves/ninja,ThiagoGarciaAlves/ninja,colincross/ninja,synaptek/ninja,ilor/ninja,mydongistiny/ninja,autopulated/ninja,automeka/ninja,jendrikillner/ninja,dendy/ninja,ninja-build/ninja,Maratyszcza/ninja-pypi,nicolasdespres/ninja,automeka/ninja,sxlin/dist_ninja,ctiller/ninja,hnney/ninja,tfarina/ninja,mohamed/ninja,lizh06/ninja,atetubou/ninja,nico/ninja,jendrikillner/ninja,mgaunard/ninja,moroten/ninja,vvvrrooomm/ninja,fuchsia-mirror/third_party-ninja,Ju2ender/ninja,sgraham/ninja,sxlin/dist_ninja,sxlin/dist_ninja,autopulated/ninja,colincross/ninja,lizh06/ninja,autopulated/ninja