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.