Sync with upstream GLFW pull request

The GLFW tree distributed with raylib has two modifications:

- GLFW_PKG_{DEPS,LIBS} are exported to PARENT_SCOPE, so we can use them
  in our pkg-config file
- An intermediary glfw_objlib target is added, so we can reexport GLFW
  symbols from libraylib.a

rglfw can fix the second point, but for Wayland usage, we would have to
replicate protocol generation, so we just leverage GLFW's existing
support instead.

To make maintenance easier, I have submitted a pull request for
including these modifications to upstream GLFW.
And to make that one easier, this patch dog-foods the modifications,
so raylib users can help find regressions. :-)

glfw/glfw#1307

src/external/glfw/CMakeLists.txt

@@ -327,10 +327,12 @@ endif()
 # Export GLFW library dependencies
 #--------------------------------------------------------------------
 foreach(arg ${glfw_PKG_DEPS})
-    set(GLFW_PKG_DEPS "${GLFW_PKG_DEPS} ${arg}" PARENT_SCOPE)
+    set(GLFW_PKG_DEPS "${GLFW_PKG_DEPS} ${arg}" CACHE INTERNAL
+        "GLFW pkg-config Requires.private")
 endforeach()
 foreach(arg ${glfw_PKG_LIBS})
-    set(GLFW_PKG_LIBS "${GLFW_PKG_LIBS} ${arg}" PARENT_SCOPE)
+    set(GLFW_PKG_LIBS "${GLFW_PKG_LIBS} ${arg}" CACHE INTERNAL
+        "GLFW pkg-config Libs.private")
 endforeach()
 
 #--------------------------------------------------------------------