Hi Guix,

the attached patch augments LDFLAGS indirectly over LIB_LINK_FLAGS with
the rpath flag for the library output of the package.  This fixes rpath
problems with /lib/libtbbmalloc_proxy.so.2 and possibly others (see
http://hydra.gnu.org/build/367038/nixlog/2/raw) as reported by Ludo.

Note: I did not run the complete test suite again (because it takes a
very long time on my machine), and when tests are enabled a couple more
libraries are created.  Since they all probably share LIB_LINK_FLAGS I
expect any problems there to also be fixed by this patch.

~~ Ricardo