Nested conditionals are hard to read if they are not indented.
We can't add arbitrary whitespace to everything in spec files,
but we CAN add spaces before %if and %define.  Use this trick,
plus a fancy sed script that rewrites a spec file into a C
file, so we can use cppi to keep our spec file nice.

For reference, the sed script converts code like:

|# RHEL-5 builds are client-only for s390, ppc
|%if 0%{?rhel} == 5
|    %ifnarch %{ix86} x86_64 ia64
|        %define client_only        1
|    %endif
|%endif

into the following for cppi:

|// # RHEL-5 builds are client-only for s390, ppc
|#if a // 0%{?rhel} == 5
|# if a // %{ix86} x86_64 ia64
|#  define client_only        1
|# endif
|#endif

and errors from 'make syntax-check' look like:
spec_indentation
cppi: mingw-libvirt.spec.in: line 130: not properly indented
maint.mk: incorrect preprocessor indentation

* libvirt.spec.in: Add some indentation to make it easier to follow
various conditionals.
* mingw-libvirt-spec.in: Likewise.
* cfg.mk (sc_spec_indentation): New syntax check to enforce it.