configure: do not require hg when configuring in an hg clone

When configuring in an hg clone, we need hg to compute the version string.
It can happen that users do not have Mercurial (eg. if they got a snapshot
rather that they did a full clone). In this case, we can still run, of
course, so simply fill the version string with a sufficiently explicit
value, that does not require hg. The date is a good candidate.

 # This file adds functions to build the C library for bare metal (IE. none)

 # Copyright 2008 Yann E. MORIN

 # Licensed under the GPL v2. See COPYING in the root of this package

 do_libc_get() {

     :

 }

 do_libc_extract() {

     :

 }

 do_libc_check_config() {

     :

 }

 do_libc_headers() {

     :

 }

 do_libc_start_files() {

     :

 }

 do_libc() {

     :

 }

 do_libc_finish() {

     :

 }