Allow building with SXEmacs.

2015-07-11  Steve Youngs  <steve@sxemacs.org>

	* build.el: Require 'build-rpt' instead of 'build-report' when
	using SXEmacs.
	Note that it is not recommended for SXEmacs users to install or
	use this package, this change is purely to allow building the
	package with SXEmacs.