Put libpgport into OBJS instead of LIBS, so that it gets included

Put libpgport into OBJS instead of LIBS, so that it gets included
into .def and .exp files automatically on Windows, AIX, and the like.
An additional benefit is that changes in libpgport files correctly
propagate to force rebuild of the backend executable.  This is my
reworking of Rocco Altier's idea, and if it breaks anything it's
definitely my fault.