Use server includes for pgport when building slonik

When building with --with-pgport slonik needs to include port.h
port.h is normally found under the server include directory but
some distributions (ie debian libpq-dev) also include it in
/usr/include/postgresql

The fix for bug 315 removed the server include paths from
all client programs including slonik.  This went too far,
the actual issue with bug 315 was src/backend failing because
it pulled in the client headers.

If we are building with pgport we will pull in the server
include directories for slonik