external: glfw: reinstate export of GLFW_PKG_{DEPS,LIBS}

We were doing this before, but it was deleted during the
last GLFW update. Readd it to fix the associated macOS CI failure.

Fixes: cd934c9f6 ("Update GLFW to 3.3.1")

src/external/glfw/CMakeLists.txt

@@ -303,10 +303,12 @@ endif()
 # Export GLFW library dependencies
 #--------------------------------------------------------------------
 foreach(arg ${glfw_PKG_DEPS})
-    set(GLFW_PKG_DEPS "${GLFW_PKG_DEPS} ${arg}")
+    set(GLFW_PKG_DEPS "${GLFW_PKG_DEPS} ${arg}" CACHE INTERNAL
+        "GLFW pkg-config Requires.private")
 endforeach()
 foreach(arg ${glfw_PKG_LIBS})
-    set(GLFW_PKG_LIBS "${GLFW_PKG_LIBS} ${arg}")
+    set(GLFW_PKG_LIBS "${GLFW_PKG_LIBS} ${arg}" CACHE INTERNAL
+        "GLFW pkg-config Libs.private")
 endforeach()
 
 #--------------------------------------------------------------------