Hi Heikki,
With PostgreSQL 8.4.11, I still face the same issue, resolved as part of the following checked-in:
http://git.postgresql.org/gitweb/?p=pldebugger.git;a=commitdiff;h=b9c756ef6cd4bdabea203905b2071d0e6b4e0d4d
I tried to resolve the issue by removing the duplicate words for the SHLIB_LINK variable in the Makefile for pldebugger.
I just changed it specifically for PostgreSQL 8.4 on OSX platform, because PostgreSQL build script may get changed in future releases (i.e. pldebugger is compiling well with PostgreSQL 9.0+ on OSX because of such modification.)
Please find the attached patch for the same.
I know - it looks ugly, but I could not think of any other solution at the moment.