configure: do not require svn

svn is needed to download eglibc from the upstream repository.
In some situations, though, it can happen that:
- the developpers' machines do not have svn
- a LAN mirror has been set-up with pre-downloaded eglibc tarballs
In this case, it makes sense not to hard-depend on svn.

So, if svn is missing, just say so, and add a comment in the
menuconfig.

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

This is NOT the official version of dialog.  This version has been

significantly modified from the original.  It is for use by the Linux

kernel configuration script.  Please do not bother Savio Lam with 

questions about this program.