Fix build flake on box2d port.
Fix build flake on box2d port.

Cmake seems to have problems building multiple targets in parallel (it spawns
two submakes on the same makefile).

BUG=none

Review URL: https://codereview.chromium.org/11871019

git-svn-id: 84b3587ce119b778414dcb50e18b7479aacf4f1d@678 7dad1e8b-422e-d2af-fbf5-8013b78bd812

yeyus/naclports,yeyus/naclports,kosyak/naclports_samsung-smart-tv,kosyak/naclports_samsung-smart-tv,dtkav/naclports,yeyus/naclports,dtkav/naclports,binji/naclports,kosyak/naclports_samsung-smart-tv,kosyak/naclports_samsung-smart-tv,kuscsik/naclports,Schibum/naclports,adlr/naclports,clchiou/naclports,kuscsik/naclports,clchiou/naclports,Schibum/naclports,kuscsik/naclports,clchiou/naclports,adlr/naclports,kuscsik/naclports,adlr/naclports,yeyus/naclports,dtkav/naclports,Schibum/naclports,adlr/naclports,kuscsik/naclports,kosyak/naclports_samsung-smart-tv,Schibum/naclports,Schibum/naclports,clchiou/naclports,yeyus/naclports,binji/naclports,binji/naclports,binji/naclports,Schibum/naclports,adlr/naclports,clchiou/naclports,yeyus/naclports,dtkav/naclports,binji/naclports,dtkav/naclports