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>

 Original patch from gentoo: gentoo/src/patchsets/gcc/4.3.1/gentoo/66_all_gcc43-pr25343.patch

 http://gcc.gnu.org/PR25343

 sniped from Debian

 2008-04-27  Roman Zippel <zippel@linux-m68k.org>

 	* config/host-linux.c (TRY_EMPTY_VM_SPACE): Define for __mc68000__.

 diff -durN gcc-4.3.1.orig/gcc/config/host-linux.c gcc-4.3.1/gcc/config/host-linux.c

 --- gcc-4.3.1.orig/gcc/config/host-linux.c	2007-08-02 12:49:31.000000000 +0200

 +++ gcc-4.3.1/gcc/config/host-linux.c	2008-06-10 14:58:28.000000000 +0200

 @@ -84,6 +84,8 @@

  # define TRY_EMPTY_VM_SPACE	0x8000000000

  #elif defined(__sparc__)

  # define TRY_EMPTY_VM_SPACE	0x60000000

 +#elif defined(__mc68000__)

 +# define TRY_EMPTY_VM_SPACE	0x40000000

  #else

  # define TRY_EMPTY_VM_SPACE	0

  #endif