(.dylib format) on Mac OS X, while not messing up loadable modules for
the backend (which are the same kind of animal as a shared library on
every other platform, but not here).  Also get the naming convention
to match OS X practice, viz libFOO.version.so not libFOO.so.version.
In support of that last, refactor code in Makefile.shlib to make it
easier to have platform-specific shlib naming conventions.
This patch is loosely based on the Fink project's current postgresql.patch.
Tested by yours truly on OS X 10.3.4; does anyone have 10.2.* to check
it on?