Make finding openssl program a configure or meson option

Make finding openssl program a configure or meson option

Various test suites use the "openssl" program as part of their setup.
There isn't a way to override which openssl program is to be used,
other than by fiddling with the path, perhaps.  This has gotten
increasingly problematic because different versions of openssl have
different capabilities and do different things by default.

This patch checks for an openssl binary in configure and meson setup,
with appropriate ways to override it.  This is similar to how "lz4"
and "zstd" are handled, for example.  The meson build system actually
already did this, but the result was only used in some places.  This
is now applied more uniformly.

Reviewed-by: Michael Paquier <michael@paquier.xyz>
Discussion: https://www.postgresql.org/message-id/flat/dc638b75-a16a-007d-9e1c-d16ed6cf0ad2%40enterprisedb.com