Let out of tree builds work

This is a patch to hopefully let out of tree builds work. Everything builds fine but I had trouble with `gacutils` when installing F Sharp, and I couldn't test this patch so I'd appreciate if this was tested before being pulled. Also, I used a simple script to add the `VPATH=@abs_srcdir` to all of the makefiles so I don't know if I accidentally added it to one of the few makefiles that don't need it. Finally, the `srcdir` assignments in the makefiles were all in immediate mode. Should the `VPATH` assignments be in immediate mode too?

Fixes https://github.com/fsharp/fsharp/issues/143 .