Patch: automatic dependencies for gcc

[Paolo Bonzini]

> * Changed how BUILD_DEPMODE is computed.  See the configure.ac patch.
>   In the Canadian case, we would try to compute dependencies for
>   '$(CC)', which failed.  Then I noticed that gcc's configure runs
>   itself in a subdir for the host -- funny, but exactly what I needed.
>   So, I extract the setting from the generated Makefile.  This part is
>   kind of ugly I suppose.

No, not that much.

Go ahead with this, but avoid changing the name of the variable between 
configure and Makefile.  Unless there is a particular reason, either 
call it BUILD_DEPMODE (which I prefer) or CC_FOR_BUILDDEPMODE in both 
places, but don't mix.

Paolo