Enable CMake policy CMP0077 where available

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 71e6ff386d)

README.md

@@ -220,6 +220,7 @@ skills.
  - Robin Leffmann
  - Glenn Lewis
  - Shane Liesegang
+ - Anders Lindqvist
  - Eyal Lotem
  - Aaron Loucks
  - Tristam MacDonald