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.

 # ia64 specific config options

 # depends on EXPERIMENTAL

 config ARCH_ia64

     select ARCH_SUPPORTS_64

     select ARCH_DEFAULT_64

     select ARCH_USE_MMU

     select ARCH_SUPPORTS_BOTH_ENDIAN

     help

       The ia64 architecture, as defined by:

         http://www.intel.com/design/itanium/arch_spec.htm