pgsql: Prevent pgcrypto from successfully compiling if no valid random

Log Message:
-----------
Prevent pgcrypto from successfully compiling if no valid random source
has been defined. Previously, pgcrypto would compile but would be
unusable.

Modified Files:
--------------
pgsql/contrib/pgcrypto:
random.c (r1.7 -> r1.8)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/contrib/pgcrypto/random.c.diff?r1=1.7&r2=1.8)