ARROW-4989: [C++] Find re2 on Ubuntu if asked to
ARROW-4989: [C++] Find re2 on Ubuntu if asked to

This allows `cmake -DRE2_SOURCE=SYSTEM` to work from a Conda environment.

Author: Antoine Pitrou <antoine@python.org>

Closes #4007 from pitrou/ARROW-4989-find-ubuntu-re2 and squashes the following commits:

60ecc3a6 <Antoine Pitrou> ARROW-4989:  Find re2 on Ubuntu if asked to

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