* automake.in (handle_single_transform_list): Use new global,

* automake.in (handle_single_transform_list): Use new global,
`%linkers_used', to store the linkers used, rather than an internal
variable.  Enables correct linker to be calculated across a group
of calls to &handle_single_transform_list.  Return only list of
objects, since linker to be used is now externally determined.
(handle_source_transform): adapted for new calling conventions of
handle_single_transform_list.  Calls resolve_linker() on a set of
all the linkers used for any prefix, rather than for each prefix in
turn.
(linkers_used): New global.
* tests/link_dist.test: New test.
* tests/Makefile.am (TESTS): Added link_dist.test.