Install all backward headers?

[Loren James Rittle <rittle at latour dot rsch dot comm dot mot dot com>]

Due to Benjamin's recent work in the area, I just noticed that way
back in May when I added some new backward headers to allow more
"traditional" g++ test cases to pass that I forgot to update
src/Makefile.am to install them.

Do we want: complex.h, iomanip.h, istream.h, ostream.h, stream.h and
streambuf.h to be installed in backward?  Or, are they just supposed
to be useful to pass old g++ test cases?

I will provide a patch (but it won't be till next week), if consensus
says that we want them installed and no one beats me to it.

Regards,
Loren