Cleanup meson.build: Also use CMake to help find OpenSSL

Hi,

This attempts to clean up the Meson build files as there is now decent support via Meson's CMake support to find OpenSSL via dependenct(), and CMake is a common item on Windows build machines (it is even an optional component for Visual Studio 2017 or later installations).

This does add a modules: section for the dependency() call so that we can be sure that we do indeed have libssl.lib (or ssleay32.lib), which CMake will check for us.

With blessings, thank you!