ARROW-5253: [C++] Fix snappy external build
ARROW-5253: [C++] Fix snappy external build

CMAKE_INSTALL_LIBDIR is not propagated thus fails on systems that uses
`lib64` (alpine and possibly redhat derivatives).

Author: François Saint-Jacques <fsaintjacques@gmail.com>

Closes #4246 from fsaintjacques/ARROW-5253-snappy-external and squashes the following commits:

f6b0bc7f8 <François Saint-Jacques> ARROW-5253:  Fix snappy external build

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