scons: Disable protobuf if pkg-config and CheckLib fails
scons: Disable protobuf if pkg-config and CheckLib fails

This patch changes the use of pkg-config such that protobuf is still
evaluated with CheckLib even if it fails. This is to allow setups
where libprotobuf is available, but not configured through
protobuf. Moreover, if CheckLib fails to use libprotobuf then all the
tracing is disabled, but scons is allowed to continue with a warning.

qizenguf/MLC-STT,zlfben/gem5,markoshorro/gem5,Weil0ng/gem5,briancoutinho0905/2dsampling,briancoutinho0905/2dsampling,SanchayanMaity/gem5,KuroeKurose/gem5,samueldotj/TeeRISC-Simulator,markoshorro/gem5,qizenguf/MLC-STT,samueldotj/TeeRISC-Simulator,cancro7/gem5,austinharris/gem5-riscv,cancro7/gem5,gem5/gem5,joerocklin/gem5,gem5/gem5,sobercoder/gem5,rallylee/gem5,yb-kim/gemV,briancoutinho0905/2dsampling,markoshorro/gem5,HwisooSo/gemV-update,gedare/gem5,kaiyuanl/gem5,TUD-OS/gem5-dtu,yb-kim/gemV,cancro7/gem5,zlfben/gem5,cancro7/gem5,rjschof/gem5,powerjg/gem5-ci-test,joerocklin/gem5,yb-kim/gemV,KuroeKurose/gem5,sobercoder/gem5,aclifton/cpeg853-gem5,TUD-OS/gem5-dtu,aclifton/cpeg853-gem5,austinharris/gem5-riscv,austinharris/gem5-riscv,zlfben/gem5,sobercoder/gem5,qizenguf/MLC-STT,Weil0ng/gem5,sobercoder/gem5,rallylee/gem5,rjschof/gem5,HwisooSo/gemV-update,SanchayanMaity/gem5,qizenguf/MLC-STT,HwisooSo/gemV-update,SanchayanMaity/gem5,rallylee/gem5,sobercoder/gem5,samueldotj/TeeRISC-Simulator,samueldotj/TeeRISC-Simulator,briancoutinho0905/2dsampling,joerocklin/gem5,KuroeKurose/gem5,joerocklin/gem5,zlfben/gem5,kaiyuanl/gem5,yb-kim/gemV,Weil0ng/gem5,kaiyuanl/gem5,Weil0ng/gem5,kaiyuanl/gem5,HwisooSo/gemV-update,gedare/gem5,HwisooSo/gemV-update,yb-kim/gemV,yb-kim/gemV,powerjg/gem5-ci-test,qizenguf/MLC-STT,aclifton/cpeg853-gem5,KuroeKurose/gem5,rallylee/gem5,TUD-OS/gem5-dtu,joerocklin/gem5,markoshorro/gem5,gem5/gem5,powerjg/gem5-ci-test,markoshorro/gem5,austinharris/gem5-riscv,SanchayanMaity/gem5,cancro7/gem5,gem5/gem5,TUD-OS/gem5-dtu,zlfben/gem5,samueldotj/TeeRISC-Simulator,KuroeKurose/gem5,austinharris/gem5-riscv,KuroeKurose/gem5,SanchayanMaity/gem5,cancro7/gem5,sobercoder/gem5,TUD-OS/gem5-dtu,austinharris/gem5-riscv,samueldotj/TeeRISC-Simulator,rallylee/gem5,kaiyuanl/gem5,powerjg/gem5-ci-test,aclifton/cpeg853-gem5,zlfben/gem5,rallylee/gem5,HwisooSo/gemV-update,TUD-OS/gem5-dtu,HwisooSo/gemV-update,Weil0ng/gem5,gedare/gem5,gedare/gem5,yb-kim/gemV,rallylee/gem5,rjschof/gem5,briancoutinho0905/2dsampling,markoshorro/gem5,powerjg/gem5-ci-test,SanchayanMaity/gem5,gem5/gem5,joerocklin/gem5,SanchayanMaity/gem5,rjschof/gem5,qizenguf/MLC-STT,austinharris/gem5-riscv,powerjg/gem5-ci-test,aclifton/cpeg853-gem5,yb-kim/gemV,KuroeKurose/gem5,briancoutinho0905/2dsampling,cancro7/gem5,Weil0ng/gem5,rjschof/gem5,markoshorro/gem5,gem5/gem5,joerocklin/gem5,gedare/gem5,kaiyuanl/gem5,rjschof/gem5,joerocklin/gem5,briancoutinho0905/2dsampling,rjschof/gem5,kaiyuanl/gem5,gem5/gem5,gedare/gem5,qizenguf/MLC-STT,TUD-OS/gem5-dtu,aclifton/cpeg853-gem5,samueldotj/TeeRISC-Simulator,aclifton/cpeg853-gem5,sobercoder/gem5,zlfben/gem5,Weil0ng/gem5,powerjg/gem5-ci-test,gedare/gem5