[Libguestfs] ocamldep -all seems to break builds on platforms without a native compiler

[Hilko Bengen]

* Richard W.M. Jones:

> Yes, also this commit doesn't actually fix the problem.  I still see
> occasional problems building StringMap.  I have reverted this commit.

Here's another patch that should enable the native-code targets only if
they can be built.

Cheers,
-Hilko