ARROW-3715: [C++]: Fix typo in gflags_ep CMake config
ARROW-3715: [C++]: Fix typo in gflags_ep CMake config

Author: Sylvain Joubert <Sylvain Joubert>

Closes #2904 from sjoubert/fix_gflags_ep_option and squashes the following commits:

5ebf6cc9 <Sylvain Joubert> : Fix typo in gflags_ep CMake config

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