Cope with cross-compiling when checking for a random-number source.

Cope with cross-compiling when checking for a random-number source.

Commit 16f96c74d neglected to consider the possibility of cross-compiling,
causing cross-compiles to fail at the configure stage unless you'd
selected --with-openssl.  Since we're now more or less assuming that
/dev/urandom is available everywhere, it seems reasonable to assume
that the cross-compile target has it too, rather than failing.

Per complaint from Vincas Dargis.  Back-patch to v14 where this came in.

Discussion: https://postgr.es/m/0dc14a31-acaf-8cae-0df4-a87339b22bd9@gmail.com