tools/Rules.mk: Properly handle libraries with recursive dependencies.

tools/Rules.mk: Properly handle libraries with recursive dependencies.

In tree libraries which link against other in tree libraries in a way
which is opaque to their callers need special handling, specifically
correct use of -Wl,-rpath-link for the recusively used libraries.

Currently this is rather simple, but up coming changes are going to
introduce transitive dependencies more than 1 step deep.

Introduce a SHDEPS idiom to contain all the recursive deps for a
library and include those in both LDLIBS (for linking) and SHLIB (for
recursive uses).

Try and document the whole thing.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>