Eliminating more recursive make invocations

[John W. Eaton]

I checked in a series of changes that eliminates calling make 
recursively for the doc, etc/icons, and test directory trees.

You may need to clean up your build tree and run the bootstrap script 
followed by configure to eliminate now obsolete Makefiles.

In doing this I once again enabled building the documentation and images 
for the documentation in parallel.  It seems to work for me, but if you 
notice problems, please report them.

I'm considering doing the same for the liboctave, libinterp, libgui, 
src, and scripts directory.  At that point, the only directory that will 
be handled by a recursive make invocation will be libgnu.

jwe