rm fatal error on glew from glfw find

Former-commit-id: 413c341a8683373b57872ea3b6d88c73a15d25b8

tutorial/cmake/FindGLEW.cmake

@@ -32,7 +32,7 @@ if(GLEW_FOUND)
   set(GLEW_SOURCES ${GLEW_INCLUDE_DIR}/../src/glew.c)
   message(STATUS "Found GLEW: ${GLEW_INCLUDE_DIR}")
 else(GLEW_FOUND)
-  message(FATAL_ERROR "could NOT find glew")
+  message(WARNING "could NOT find glew")
 endif(GLEW_FOUND)
 
 MARK_AS_ADVANCED(GLEW_INCLUDE_DIR)