Add GLFW source tree to src/external

We need the CMake stuff for wayland configuration.
Otherwise, we would have to replicate that ourselves.
This is the full 7ef34eb06de54dd9186d3d21a401b2ef819b59e7 tree except
for tests/ examples/ and docs/

src/external/glfw/CMake/modules/FindVulkan.cmake

@@ -0,0 +1,46 @@
+# Find Vulkan
+#
+# VULKAN_INCLUDE_DIR
+# VULKAN_LIBRARY
+# VULKAN_FOUND
+
+if (WIN32)
+    find_path(VULKAN_INCLUDE_DIR NAMES vulkan/vulkan.h HINTS
+        "$ENV{VULKAN_SDK}/Include"
+        "$ENV{VK_SDK_PATH}/Include")
+    if (CMAKE_SIZEOF_VOID_P EQUAL 8)
+        find_library(VULKAN_LIBRARY NAMES vulkan-1 HINTS
+            "$ENV{VULKAN_SDK}/Lib"
+            "$ENV{VULKAN_SDK}/Bin"
+            "$ENV{VK_SDK_PATH}/Bin")
+        find_library(VULKAN_STATIC_LIBRARY NAMES vkstatic.1 HINTS
+            "$ENV{VULKAN_SDK}/Lib"
+            "$ENV{VULKAN_SDK}/Bin"
+            "$ENV{VK_SDK_PATH}/Bin")
+    else()
+        find_library(VULKAN_LIBRARY NAMES vulkan-1 HINTS
+            "$ENV{VULKAN_SDK}/Lib32"
+            "$ENV{VULKAN_SDK}/Bin32"
+            "$ENV{VK_SDK_PATH}/Bin32")
+        find_library(VULKAN_STATIC_LIBRARY NAMES vkstatic.1 HINTS
+            "$ENV{VULKAN_SDK}/Lib32"
+            "$ENV{VULKAN_SDK}/Bin32"
+            "$ENV{VK_SDK_PATH}/Bin32")
+    endif()
+elseif (APPLE)
+    find_library(VULKAN_LIBRARY vulkan.1 HINTS
+        "$ENV{VULKAN_SDK}/macOS/lib")
+    find_path(VULKAN_INCLUDE_DIR NAMES vulkan/vulkan.h HINTS
+        "$ENV{VULKAN_SDK}/macOS/include")
+else()
+    find_path(VULKAN_INCLUDE_DIR NAMES vulkan/vulkan.h HINTS
+        "$ENV{VULKAN_SDK}/include")
+    find_library(VULKAN_LIBRARY NAMES vulkan HINTS
+        "$ENV{VULKAN_SDK}/lib")
+endif()
+
+include(FindPackageHandleStandardArgs)
+find_package_handle_standard_args(Vulkan DEFAULT_MSG VULKAN_LIBRARY VULKAN_INCLUDE_DIR)
+
+mark_as_advanced(VULKAN_INCLUDE_DIR VULKAN_LIBRARY VULKAN_STATIC_LIBRARY)
+