Support interprocedural optimization override

This policy applies the CMAKE_INTERPROCEDURAL_OPTIMIZATION override to
the GLFW target.

Fixes #1753.

CMakeLists.txt

@@ -8,6 +8,10 @@ if (POLICY CMP0054)
     cmake_policy(SET CMP0054 NEW)
 endif()
 
+if (POLICY CMP0069)
+    cmake_policy(SET CMP0069 NEW)
+endif()
+
 if (POLICY CMP0077)
     cmake_policy(SET CMP0077 NEW)
 endif()