recent build breakage

[sfeam (E. Merritt) <eam...@gm...>]

The rearrangement of windows documentation seems to have broken the
build scripts.  Here's what I get from "make distcheck"

make[2]: Entering directory `/home/merritt/cvs/gnuplot-cvs/gnuplot-4.5/_build/docs'
make[2]: *** No rule to make target `windows/doc2html.c', needed by `distdir'.  Stop.
make[2]: Leaving directory `/home/merritt/cvs/gnuplot-cvs/gnuplot-4.5/_build/docs'
make[1]: *** [distdir] Error 1
make[1]: Leaving directory `/home/merritt/cvs/gnuplot-cvs/gnuplot-4.5/_build'
make: *** [distcheck] Error 1

I have updated the source tree using "cvs update -d", and yes there is a file 
windows/doc2html.c in my starting source. The problem is that the new subdirectory
does not get included in the build package.

Makefile.in instead refers to a non-existent directory  .../docs/wxhelp
although it doesn't try to include this directory in the build package either.

So which is it - .../docs/windows or .../docs/wxhelp?   Or both?
In any case, the Makefile needs to be updated to know about this
new directory structure.

	Ethan