Simplify build on macOS (#35570)

Simplify build on macOS (#35570)

* Check for OpenSSL (and pkg-config) in the check_prereqs() function
* Simplify OpenSSL installation instructions
* Automatically pick the OpenSSL version installed by Homebrew by exporting the proper PKG_CONFIG_PATH environment variable
* Improve the error message if cmake can't find OpenSSL

The previous instructions were asking the user to add symbolic links inside  /usr/local/lib/ and /usr/local/lib/pkgconfig/ for OpenSSL related files. There is no need for a complicated setup with symbolic links when the PKG_CONFIG_PATH environment variable points to the right pkgconfig, which is now done in build-commons.sh.

Co-Authored-By: Jan Kotas <jkotas@microsoft.com>
Co-Authored-By: Jeremy Barton <jbarton@microsoft.com>