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>

mainmenu "The crosstool-NG configuration menu"

source "config/configure.in"

source "config/backend.in"

source "config/global.in"

source "config/target.in"

source "config/toolchain.in"

source "config/kernel.in"

source "config/binutils.in"

source "config/cc.in"

source "config/libc.in"

source "config/debug.in"

source "config/companion_libs.in"

source "config/companion_tools.in"

source "config/test_suite.in"