Claudio authored Aug 06, 2019 and Brad King committed Aug 06, 2019

Fix logic added by commit a7d85386 (FindGLEW: Update implementation,
2019-03-13, v3.15.0-rc1~375^2~1) on macOS.  macOS is recognized as both
UNIX and APPLE.  Consequently, the library suffix for shared and static
library was set, respectively, as `.so` and `.a`, just like UNIX systems.
Fix this by properly checking the OS type.

Fixes: #19542