[libstdc++] Fix _GLIBCPP_VERSION

[B. Kosnik]

> missed an occurance of release_VERSION, and as such, caused
> _GLIBCPP_VERSION to be set to an empty string.  Now that the library's
> release version is taken directly from the compiler rather than being
> maintained seperately,_GLIBCPP_VERSION is yet another variation of
> __GNUC__, __GNUC_MINOR__, and __VERSION__.  Reinventing the wheel, but
> with prettier spokes.  :-)

Hmmm. Whoops. Sorry about this fallout.

> Benjamin's patch also had code to discover the compiler version from
> the sources.  I've replaced that with a call to the compiler instead
> (simpler code), and shuffled the configury script fragments a bit.  So
> in addition to using the version to determine the header file
> directory, we also use it to create _GLIBCPP_VERSION.

Nice. You might want to consider moving this up to gcc/configure.in and
../config.if as well.

thanks,
benjamin