Centralized shared-library build knowledge in a new file,

src/Makefile.shlib.  Updated all the makefiles that try to build shlibs
to include that file instead of having duplicate (and mostly incomplete)
copies of shared-library options.  It works on HPUX, a lot better than it
did before in fact, but there's a chance I broke some other platforms.
At least now you only have to fix one place not six...