scripts/functions: cvs retrieval first tries the mirror for tarballs

The cvs download helper looks for the local tarballs dir to see if it
can find a pre-downloaded tarball, and if it does not find it, does
the actual fetch to upstream via cvs.

In the process, it does not even try to get a tarball from the local
mirror, which can be useful if the mirror has been pre-populated
manually (or with a previously downloaded tree).

Fake a tarball get with the standard tarball-download helper, but
without specifying any upstream URL, which makes the helper directly
try the LAN mirror.

Of course, if no mirror is specified, no URL wil be available, and
the standard cvs retrieval will kick in.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

 --- gcc-4.2.1-stock/gcc/config/alpha/linux-unwind.h	2005-06-24 20:22:41.000000000 -0500

 +++ gcc-4.2.1/gcc/config/alpha/linux-unwind.h	2007-09-01 20:21:34.000000000 -0500

 @@ -29,6 +29,9 @@ Boston, MA 02110-1301, USA.  */

  /* Do code reading to identify a signal frame, and set the frame

     state data appropriately.  See unwind-dw2.c for the structs.  */

 +/* Don't use this if inhibit_libc is set

 +   The build for this target will fail trying to include missing headers */

 +#ifndef inhibit_libc

  #include <signal.h>

  #include <sys/ucontext.h>

 @@ -80,3 +83,4 @@ alpha_fallback_frame_state (struct _Unwi

    fs->retaddr_column = 64;

    return _URC_NO_REASON;

  }

 +#endif /* inhibit_libc */