This will let higher-level projects override GLFW CMake options with
normal variables instead of having to use cache variables.

This means with CMake 3.13 and later you can now do:

set(GLFW_BUILD_TESTS ON)
add_subdirectory(path/to/glfw)

Instead of the more verbose:

set(GLFW_BUILD_TESTS ON CACHE BOOL "" FORCE)
add_subdirectory(path/to/glfw)

(cherry picked from commit 71e6ff386d9a3e0cc7c069be9b0ee1a9ac738647)