Configuration Question

Greetings:

Is there a way to get hold of an environment variable such that it can be
referenced in postgresql.conf? In particular, I'd like to be able to point
dynamic_library_path to an environment variable defined at the system level
as in dynamic_library_path = '$SOURCE:$libdir'.

master=# select version();
                                                version
--------------------------------------------------------------------------------------------------------
 PostgreSQL 8.3.3 on i686-redhat-linux-gnu, compiled by GCC gcc (GCC) 4.1.2
20070626 (Red Hat 4.1.2-14)

TIA
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