[build] Update BUILD.gn for glfw3.3 transition.

BLD-564 #comment

Change-Id: I54c6e16c34a9fb172f8d2c6af783c192f52f1be5
diff --git a/BUILD.gn b/BUILD.gn
index 81a74fa..0166f0d 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -26,11 +26,13 @@
   ]
   sources = [
     "deps/getopt.c",
-    "deps/glad.c",
+    "deps/glad_vulkan.c",
     "src/context.c",
+    "src/egl_context.c",
     "src/init.c",
     "src/input.c",
     "src/monitor.c",
+    "src/osmesa_context.c",
     "src/vulkan.c",
     "src/window.c",
   ]
@@ -54,16 +56,15 @@
       "src/cocoa_time.c",
       "src/cocoa_window.m",
       "src/nsgl_context.m",
-      "src/posix_tls.c",
+      "src/posix_thread.c",
     ]
   } else if (is_linux) {
     defines += [ "_GLFW_X11" ]
     sources += [
-      "src/egl_context.c",
       "src/glx_context.c",
       "src/linux_joystick.c",
+      "src/posix_thread.c",
       "src/posix_time.c",
-      "src/posix_tls.c",
       "src/x11_init.c",
       "src/x11_monitor.c",
       "src/x11_window.c",