FindGLEW: Fix macOS library suffix selection

Fix logic added by commit a7d8538 (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

Author: claudiofantacci

Modules/FindGLEW.cmake

@@ -72,9 +72,9 @@ endif()
 
 
 function(__glew_set_find_library_suffix shared_or_static)
-  if(UNIX AND "${shared_or_static}" MATCHES "SHARED")
+  if((UNIX AND NOT APPLE) AND "${shared_or_static}" MATCHES "SHARED")
     set(CMAKE_FIND_LIBRARY_SUFFIXES ".so" PARENT_SCOPE)
-  elseif(UNIX AND "${shared_or_static}" MATCHES "STATIC")
+  elseif((UNIX AND NOT APPLE) AND "${shared_or_static}" MATCHES "STATIC")
     set(CMAKE_FIND_LIBRARY_SUFFIXES ".a" PARENT_SCOPE)
   elseif(APPLE AND "${shared_or_static}" MATCHES "SHARED")
     set(CMAKE_FIND_LIBRARY_SUFFIXES ".dylib;.so" PARENT_SCOPE)