Small cleanup of OpenSSL/LibreSSL CMake

[fpagliughi]

This implements the fix for the SSL CMake issue I commented on in #1451, here

@marcfir

[fpagliughi]

This also changes the CMake output to look a little more consistent (nicer, in my opinion):

-- CMake system name: Linux
-- Timestamp is 2024-07-20T04:11:46Z
-- Found OpenSSL: /usr/lib/x86_64-linux-gnu/libcrypto.so (found version "3.0.2")  
-- Using OpenSSL with headers at /usr/include, lib /usr/lib/x86_64-linux-gnu/libssl.so
-- Configuring done

[fpagliughi]

I also just pushed a 2nd commit into the PR.

Using CMAKE_SOURCE_DIR is not correct to find files relative to the project's source tree. It is the top-level directory in the build, and thus will change if the project is bring built from a subdirectory, such as using a Git submodule. (Thus it will be the directory of the parent project, not the Paho C project directory).

Using PROJECT_SOURCE_DIR will keep it pinned to the Paho C sources even if built as a subdirectory. (Using CMAKE_CURRENT_SOURCE_DIR would have worked here as well).