The default C++ headers install place

[Kai Ruottu]

This has been changed recently but why?  The "Install" docs in
'gcc.gnu.org' however still says :

--with-gxx-include-dir=dirname
     Specify the installation directory for G++ header files. The default
     is prefix/include/c++/version.

Writing 'make install' in gcc-4.3.3 really didn't do this but put the
headers into '$prefix/$target/include/c++/$version' !  So they became
"target and version dependent" instead of the earlier "version
dependent". Only the '$prefix/include/c++/$version/$target' was that
target dependent in them...

So now one MUST specify the old default if one wants the build to
follow the documentation and the old learned convention :(