disable build of glew on macosx

Former-commit-id: aaf998ef33952d00b81adae9db22274a236d85ee

shared/cmake/CMakeLists.txt

@@ -222,7 +222,7 @@ if(LIBIGL_WITH_VIEWER)
   endif()
 
   ### GLEW for linux and windows
-  if(UNIX OR (WIN32 AND NOT LIBIGL_WITH_NANOGUI)) ### Compile glew if needed
+  if((UNIX AND NOT APPLE) OR (WIN32 AND NOT LIBIGL_WITH_NANOGUI)) ### Compile glew if needed
     set(GLEW_INSTALL OFF CACHE BOOL " " FORCE)
     add_subdirectory("${NANOGUI_DIR}/ext/glew" "glew")
   endif()