Shared library building features from Ego.

Shared library building features from Ego.

(1) SO_RUNDIR can be used to choose the place in the run tree where
    the .so should be installed.
(2) lib-flags is used, so shared libs can directly depend on other shared
    libs if needed. (This is useful for building shared libs that do not
    use pkgconfig themselves -- e.g., Perl/Python modules.)