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

The svn 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 svn.

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 svn retrieval will kick in.

Reported-by: ANDY KENNEDY <ANDY.KENNEDY@adtran.com>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

Original patch from: mpf_set_str.c.4.diff

-= BEGIN original header =-

-= END original header =-

diff -durN gmp-4.2.4.orig/mpf/set_str.c gmp-4.2.4/mpf/set_str.c

--- gmp-4.2.4.orig/mpf/set_str.c	2008-08-25 16:11:37.000000000 +0200

+++ gmp-4.2.4/mpf/set_str.c	2009-03-08 18:36:16.000000000 +0100

@@ -137,7 +137,12 @@

       c = (unsigned char) *++str;

     }

+  /* Default base to decimal.  */

+  if (base == 0)

+    base = 10;

+

   exp_base = base;

+

   if (base < 0)

     {

       exp_base = 10;

@@ -165,10 +170,6 @@

 	return -1;

     }

-  /* Default base to decimal.  */

-  if (base == 0)

-    base = 10;

-

   /* Locate exponent part of the input.  Look from the right of the string,

      since the exponent is usually a lot shorter than the mantissa.  */

   expptr = NULL;