http://grass.itc.it/statsgrass/r_and_dbms.html

don't forget

Sys.setenv("PG_LIB_DIR"="/usr/local/pgsql/lib")
Sys.setenv("PG_INCLUDE_DIR"="/usr/local/pgsql/include")

because for some reason 'could not find your PostgreSQL client libraries', so set PG_LIB_DIR in your environment to the library path. That's what you are doing.