Allow disabling of gdbserver build (--enable-gdbserver=yes/no/auto).

Allow disabling of gdbserver build (--enable-gdbserver=yes/no/auto).

This patch adds a new --enable-gdbserver=yes/no/auto command-line switch
in gdb/configure.  The primary purpose is to allow a user to disable the
build & install of gdbserver when not desired.  It also allows the user
to request gdbserver in which case the configure script will abort if
automatic building of gdbserver is not supported for that configuration.

The default keeps things as is: We automatically build gdbserver if
building for a native configuration and if gdbserver is supported for
that configuration.

gdb/ChangeLog:

        * configure.ac: Add support for --enable-gdbserver.
        * configure: Regenerate.

At AdaCore, this is mostly intended for ia64-linux, where we currently
do not want to distribute gdbserver.  This will be useful in the context
of I417-033, where we are now using "make install" to determine what
needs to be packaged inside the GNAT Pro package.