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