scl_prefix vs _scl_prefix

[Toshio Kuratomi]

Here's a relatively [ ;-) ] non-controversial change needed to the draft.
Right now we have both _scl_prefix and scl_prefix macros.  We need to rename
one of those.  Names that differ only in punctuation are confusing.  It
looks like _scl_prefix is used less in the draft.  So that's probably the
candidate to rename.  Perhaps something like %{_scldir} to be similar to
%{_libdir}, %{_sysconfdir}, etc?

-Toshio