Only warn on errors in release mode
Only warn on errors in release mode

This improves development iterations when working in debug mode as you
don't have to fix up warnings to get the code to compile. Once a feature
is complete you can then run a release build and fix up all the warnings.

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