Add a new print-output build variable for the doc project, 'RLE'.

If defined, EPS files are run length encoded before being integrated
into the PostScript output.

"make book.ps" currently generates a 96 megabyte file.
"make RLE=1 book.ps" generates a 16 megabyte file.

If we added a tool to use better (LZW) compression for the eps
screenshots and such, then we could reduce this number further.