libstdc++47: Rework to prevent regeneration during install

libstdc++47: Rework to prevent regeneration during install

The installincludes target regenerated files such as c++config.h during
the install phase.  It may be a minor bug as "bits/c++config.h:" would
trigger it but "c++config.h:" without the subdirectory wouldn't.  In any
case, generating it and two other headers to the local working bits
directory wasn't necessary as installincludes target installs them in an
accessible location.  Reworking the makefiles prevents a write operation
during installworld.

Reported-by: dillon