Thanks Tom and Reece for replying. It turns out that the
cryptolib/cryptsrc/random source combination I specified --
openssl/system/openssl -- was the problem; changing the combination to
openssl/builtin/openssl, "solved" the problem.
~/contrib/pgcrypto/Makefile snippet:
# either 'builtin', 'mhash', 'openssl'
cryptolib = openssl # builtin
# either 'builtin', 'system'
cryptsrc = system # builtin <<< problem; revert to 'builtin'
# Random source, preferred order:
# 'dev' - read from random device
#
# 'openssl' - use openssl PRNG.
# Note that currently pgcrypto does not do any
# entropy feeding to it
# This works ofcouse only with cryptolib = openssl
#
# 'silly' - use libc random() - very weak
random = openssl # silly