I think the postgresql-server-dev packages should not need libpq-dev at all, at least for the header files. (I think the reason for the dependency might be something to do with pg_config.) At least if you compile a server-side module, you should be able to compile that using only -I$(pg_config --includedir-server), which does not refer to any libpq-dev files.
I agree that the dependency is at least weird, if not a bug, but could you provide an example where an actual build failure happens because of this? -- To UNSUBSCRIBE, email to [EMAIL PROTECTED] with a subject of "unsubscribe". Trouble? Contact [EMAIL PROTECTED]