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 with some of the work I have been doing, 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.