solib version support

[ghost]

Presently, adding version to the name of shared library in unix requires something like:

import virtual-target ;

rule add-version ( name : type ? : property-set )
{
    if $(type) = SHARED_LIB &&
              ( ! ( [ $(property-set).get <target-os> ] in windows cygwin darwin aix ) &&
                  ! ( [ $(property-set).get <toolset> ] in pgi ) )
   {
      local result = [ virtual-target.add-prefix-and-suffix $(name) : $(type) :
                 $(property-set) ] ;
      result = $(result).1.2.3 ;
      return $(result) ;
   }
}

Which is too complex. Maybe, 'solib-version' property is in order.