Abort configure immediately if building GDB in tree

Abort configure immediately if building GDB in tree

The move of gnulib to the top src directory is causing the GDB build
to break if configured in tree. We hope to lift that limitation at
some point but, in the meantime, this commit allows us to abort
the initial configure right away with a clear error message should
the user attempt to build in tree.

ChangeLog:

        * configure.ac: Abort the build with an error if trying to build
        GDB in tree.
        * configure: Regenerate.