Major change in C++ doc strings

[John W. Eaton]

I checked in the following changeset so now docstrings in C++ files are 
comments rather than C strings.

  http://hg.savannah.gnu.org/hgweb/octave/rev/112b20240c87

This is quite a disruptive change, but I think it is worth doing because 
it will make writing docstrings much easier since you will no longer 
need to write \n\ at the end of every line, or \" to escape quotation 
marks inside the docstring text.

I've also changed the build system so that it doesn't use the C 
preprocessor to generates intermediate .df files that contain info about 
DEFUN functions.  Any info about DEFUN functions that is needed by the 
build system is extracted directly from the source files.

See also this bug report:

  https://savannah.gnu.org/bugs/?func=detailitem&item_id=36524

jwe