Polish shared library build to reduce number of special hacks. In

Polish shared library build to reduce number of special hacks.  In
particular, allow linking with arbitrary commands rather than only $(AR) or
$(LD), and treat C++ without hacks.

Add option to disable shared libraries.  This takes the place of the
BSD_SHLIB variable.  The regression test driver ignores the plpgsql test
if there are no shared libraries available.