Relax the precompiled header performance criterion to a 15% improvement, and print useful information if we fail to meet that.
Relax the precompiled header performance criterion to a 15% improvement,
and print useful information if we fail to meet that.

timj/scons,timj/scons,timj/scons,timj/scons,timj/scons,andrewyoung1991/scons,timj/scons,andrewyoung1991/scons,andrewyoung1991/scons,andrewyoung1991/scons,andrewyoung1991/scons,andrewyoung1991/scons,timj/scons,timj/scons,timj/scons,andrewyoung1991/scons,andrewyoung1991/scons,andrewyoung1991/scons