We need to find a better way to fix this.

We need to find a better way to fix this.
The problem arises when a target depends on a ggo.h but does not depends on the
ggo.c.
When a target depends on a ggo.c it is explicitly listed in its dependencies
including the custom command that generates it.
When the ggo.h is include in a cxx included in a target CMake has no way to
know it and does not include the custom command that generates it.

In this patch I manually add the dependency between the cxx and the ggo.h.