ARROW-4898: [C++] Old versions of FindProtobuf.cmake use ALL-CAPS for variables
ARROW-4898: [C++] Old versions of FindProtobuf.cmake use ALL-CAPS for variables

Author: Korn, Uwe <Uwe.Korn@blue-yonder.com>

Closes #3919 from xhochy/ARROW-4898 and squashes the following commits:

dea6b6f95 <Korn, Uwe> ARROW-4898:  Old versions of FindProtobuf.cmake use ALL-CAPS for variables

icexelloss/arrow,wesm/arrow,cpcloud/arrow,xhochy/arrow,xhochy/arrow,xhochy/arrow,laurentgo/arrow,icexelloss/arrow,majetideepak/arrow,xhochy/arrow,cpcloud/arrow,laurentgo/arrow,cpcloud/arrow,xhochy/arrow,itaiin/arrow,cpcloud/arrow,laurentgo/arrow,kou/arrow,renesugar/arrow,apache/arrow,kou/arrow,kou/arrow,kou/arrow,renesugar/arrow,apache/arrow,itaiin/arrow,kou/arrow,apache/arrow,icexelloss/arrow,laurentgo/arrow,apache/arrow,xhochy/arrow,itaiin/arrow,laurentgo/arrow,majetideepak/arrow,itaiin/arrow,renesugar/arrow,cpcloud/arrow,laurentgo/arrow,icexelloss/arrow,majetideepak/arrow,icexelloss/arrow,majetideepak/arrow,laurentgo/arrow,kou/arrow,majetideepak/arrow,majetideepak/arrow,laurentgo/arrow,laurentgo/arrow,cpcloud/arrow,kou/arrow,majetideepak/arrow,laurentgo/arrow,itaiin/arrow,majetideepak/arrow,kou/arrow,icexelloss/arrow,itaiin/arrow,cpcloud/arrow,icexelloss/arrow,itaiin/arrow,wesm/arrow,xhochy/arrow,cpcloud/arrow,majetideepak/arrow,renesugar/arrow,xhochy/arrow,cpcloud/arrow,xhochy/arrow,renesugar/arrow,renesugar/arrow,wesm/arrow,apache/arrow,kou/arrow,xhochy/arrow,renesugar/arrow,itaiin/arrow,itaiin/arrow,majetideepak/arrow,wesm/arrow,apache/arrow,laurentgo/arrow,laurentgo/arrow,itaiin/arrow,icexelloss/arrow,apache/arrow,renesugar/arrow,renesugar/arrow,icexelloss/arrow,majetideepak/arrow,icexelloss/arrow,icexelloss/arrow,majetideepak/arrow,xhochy/arrow,apache/arrow,apache/arrow,wesm/arrow,wesm/arrow,xhochy/arrow,cpcloud/arrow,apache/arrow,icexelloss/arrow,wesm/arrow,xhochy/arrow,wesm/arrow,kou/arrow,renesugar/arrow,cpcloud/arrow,itaiin/arrow,wesm/arrow,apache/arrow,renesugar/arrow,renesugar/arrow,kou/arrow,wesm/arrow,wesm/arrow,renesugar/arrow,cpcloud/arrow,xhochy/arrow,laurentgo/arrow,majetideepak/arrow,itaiin/arrow,majetideepak/arrow,kou/arrow,itaiin/arrow,majetideepak/arrow,itaiin/arrow,kou/arrow,wesm/arrow,apache/arrow,apache/arrow,wesm/arrow,itaiin/arrow,renesugar/arrow,kou/arrow,apache/arrow,xhochy/arrow,cpcloud/arrow,renesugar/arrow,laurentgo/arrow,wesm/arrow,laurentgo/arrow,icexelloss/arrow,icexelloss/arrow,cpcloud/arrow