Replace glad and the Vulkan SDK with glad2

This removes all dependencies from the GLFW test programs on the Vulkan
SDK.

It also removes support for linking the GLFW shared library (dynamic
library, DLL) against the Vulkan loader static library.

docs/news.dox

@@ -341,6 +341,20 @@ during compilation of GLFW will still build but it will have no effect and the
 default behaviors will be used.
 
 
+@subsubsection vulkan_sdk_33 LunarG Vulkan SDK dependency
+
+The GLFW test programs that previously depended on the LunarG Vulkan SDK now
+instead uses a Vulkan loader generated by
+[glad2](https://github.com/Dav1dde/glad).  This means the GLFW CMake files no
+longer look for the Vulkan SDK.
+
+Existing CMake projects that depended on the Vulkan SDK cache variables from
+GLFW will need to call `find_package(Vulkan)` themselves.  CMake 3.7 and later
+already comes with a
+[Vulkan find module](https://cmake.org/cmake/help/latest/module/FindVulkan.html)
+similar to the one GLFW previously included.
+
+
 @subsubsection lib_suffix_33 CMake option LIB_SUFFIX
 
 The `LIB_SUFFIX` CMake option has been removed.  GLFW now uses the