postgresql: fix ZIC configure variable assignment

The 'ZIC=$$(ZIC)' assignment is seen as 'ZIC=$(ZIC)' by the shell, that
interprets that as command substitution causing an error like:

/bin/sh: ZIC: command not found

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>