There is problem with Digital's cc/ld, which doesn't
understand this combination of command line parameters:

  cc $CFLAGS $LDFLAGS $LIBS -o conftest conftest.c

for example:
  cc ... -o conftest -lpq conftest.c
in such cases it reports about unresolved symbols, that
really is in library.

You need to change configure due to this feature.

This error appears, for example, when configure checks
for PQcmdTuples, where you are using
  LDFLAGS="$LDFLAGS $PGSQL_FLAGS $PGSQL_LIBS"
but really need
  LDFLAGS="$LDFLAGS $PGSQL_FLAGS"
  LIBS="$LIBS $PGSQL_LIBS"
with all subsequent changes (save/restore old_LIBS)