Configury changes to build libstdc++ headers

[Stephen M. Webb]

>
> We need to get rid of explicit rules to install headers if at all
> possible.
> Look at what libsupc++ does to install headers, and try to match it. It's
> the most correct solution.
>
> headers = \
> 	exception new typeinfo cxxabi.h exception_defines.h
> glibcppinstalldir = @gxx_include_dir@
> glibcppinstall_HEADERS = $(headers)

I'll see what I can do, but automake deals well with
neither subdirectories nor include files nor multiple
files with the same name, all of which are involved here.
I also wanted to stay away from using make extensions
(like VPATH) for improved portability.

Of course, if I can take advantage of someone else'e
workaround or solution, I will do do cheerfully.  Back
to the compiling board....

Stephen M. Webb