Circumvent a bug in automake (at least found in v1.11)

Circumvent a bug in automake (at least found in v1.11)
If lib*_la_SOURCES are defined in a if/endif alternative, it works for
  compilation, but the dist target uses the first variante of the
  variable, regardless of whether this is the used variante or not.
So, use yet another name in the alternative and define lib*_la_SOURCES
  only once, out of the alternatives

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/simgrid/simgrid/trunk@6861 48e7efb5-ca39-0410-a469-dd3cf9ba447f