Add fix for CMake OpenSSL root parameter

[Mwni]

The documentation incorrectly instructs to use OPENSSL_ROOT instead of OPENSSL_ROOT_DIR when creating build files with CMake.

This change copies the variable OPENSSL_ROOT to OPENSSL_ROOT_DIR whenever only the former is defined. This is to

• statisfy the fixed requirement by CMake of calling the path OPENSSL_ROOT_DIR
• fix the documentation
• stay in conformity with the naming convention of BOOST_ROOT.

[thejohnfreeman]

What documentation instructs the use of OPENSSL_ROOT? Can you link it, please?

[Mwni]

https://github.com/XRPLF/rippled/tree/develop/Builds/VisualStudio2019#build-using-visual-studio-integrated-cmake
The VS 2017 documentation has moved and changed since when this PR was filed.

[thejohnfreeman]

I see. That documentation is slated to be removed in #4381. Soon thereafter I plan to come through this CMake code and remove the variable shuffling entirely. Our CMake should call find_package and leave the discovery up to whatever Find Module / Package Configuration File the builder decides to use, whether generated by Conan, supplied by CMake, or written by themselves.

[Mwni]

Very well. I'm closing this.