How can White defend his dark square bishop against the black knight on g6?

I was speaking with a friend, who told me that he always plays against London System this way:

1.d4 d5 2.Bf4 e6 3. e3 Ne7 4. Nd2 Ng6

Now, if White plays 5. Bg3, Black attacks with h5, threatening h6.
What should White do about his Bishop? What are the general ideas in this position for White?

Hello @bspinto

I don't think you should be worried about this variation. The knight is in almost every variation misplaced on g6. You can always win a move by playing Bd3, forcing black to retreat his knight to e7.
I'll give you some concrete lines:
1.d4 d5 2.Bf4 e6 3.e3 Ne7 4.Nf3 Ng6 5.Bg3 h5 6.h3 h4 7.Bh2 Bd6 8.Bd3 Bxh2 9.Nxh2 Ne7 10.0-0 Now the pawn on h4 is just weak. First you finish you development, after you did that you can try to open the position with c4 or e4.
This is a link to the variation:
http://view.chessbase.com/cbreader/2018/4/20/Game592180203.html
Message me if you have any questions!

Greetings,
Funbeat

Hello @funbeat

Thank you so much for the explanations! :)
I was worried about to lose the dark square bishop early (by the g6-knight or d6-bishop) and mess up the white position.
In fact, the h4-pawn is weak, and White is better in development in your main line. But I have a question about de 4.c4 variation: Isn't suppose to white to build the pyramid by d4, e3 and c3? In that case (4.c4), would we still playing the London System?

Once again, thank you. :)

Greetings,
bspinto

Hello @bspinto

You're right. After c4 it's not a London System anymore. However I think white should strike immediately in the center in this line, because played some dubious moves.
I'm sure that you are not going to mess up the white position in this line :)

Greetings,
Funbeat