libstdc++ in a combined tree

[DJ Delorie <dj at redhat dot com>]

Another one like libssp.

In libstdc++-v3's configure.ac, we see this:

# This depends on GLIBCXX CHECK_LINKER_FEATURES, but without it assumes no.
GLIBCXX_ENABLE_SYMVERS([yes])

The comment lies.  If we haven't yet checked the linker features, it
will check them, and configure will fail in a combined build because
newlib/libgloss hasn't yet been installed.

Since we already check for cross compiling, I've been using this
conditional instead:

if test x"$gcc_no_link" = xyes; then true; else
  # This will run GLIBCXX CHECK_LINKER_FEATURES, so we can't use it if we can't
  # build executables.
  GLIBCXX_ENABLE_SYMVERS([yes])
fi

but of course this will drop support for cross compiled linux targets
having symbol versioning.  Suggestions?