Makefile.am: updates on linkback scheme
Several changes:

* Change the definition of $(linkback) to link to libfabric.la in the
  build tree (i.e., let Libtool figure out what the right directory
  is)
* Move $(linkback) and other -l libraries to provider DL LIBADD lines
  -- they do not belong in LDFLAGS
* Add $(linkback) to provider DL DEPENDENCIES to guarantee that
  libfabric is built before the provider DLs (e.g., in a parallel
  build)

Signed-off-by: Jeff Squyres <jsquyres@cisco.com>
Merge pull request #207 from jsquyres/pr/provider-helper-functions

Fix parallel build/linkback scheme