Q: Forcing a -Wl,-rpath arg to static lib users

Hello folks! I've been using libtool (via autotools) for a while, but have
been unable to find a clean solution to the following problem:

I have a static `liblocal.la` (built in my build system) that links to a
third-party shared lib. Obviously I can use my own lib to express that
dependency:

liblocal_la_LIBADD = -lexternal -L/somewhere/lib

The missing bit of functionality is -Wl,-rpath,/somewhere/lib that I need
to impose onto the leaf-level executables that link my local lib.
Unfortunately these attempts do not work:
 1. Cannot add that flag to _LIBADD as libtool complains: "...belong in
'liblocal_la_LDFLAGS"
 2. Cannot add that flag to _LDFLAGS as it does not carry all the way to
the leaf-level executable

I have found that I can manually jam the flag into the
`inherited_linker_flags` variable inside the .la file and that gives me the
desired result... but I don't know how to do that via Makefile.am.

I'd appreciate any and all hints/suggestions.

Thanks!
Oleg.