[libvirt] [PATCH] build: enforce preprocessor indentation

[Jim Meyering]

Eric Blake wrote:
> Since cppi is not part of Fedora Core 12, the check is conditional:
> without cppi, running 'make syntax-check' merely warns:
>
> $ make sc_preprocessor_indentation
> preprocessor_indentation
> maint.mk: skipping test sc_preprocessor_indentation: cppi not installed
>
> * cfg.mk (sc_preprocessor_indentation): New syntax-check rule.
> (preprocessor_exempt): New macro, with first exemption.

Perfect.  Thank you.
I've pushed it.