[configure] Use hg, instead of svn, to compute the version string

Now we moved from Subversion to Mercurial, use the hg command to
compute the version string.

Manually backported from 1409:7264ce426be4 plus 1430:933eee133cbf
as too much changes have been made to ./configure in the meantime.

 # Compute x86_64-specific values

 CT_DoArchTupleValues() {

     # The architecture part of the tuple:

     CT_TARGET_ARCH="${CT_ARCH}"

 }