Fix RPATH related issues

[speth]

Changes proposed in this pull request

• Set the Cantera shared library to use RPATH on macOS. Previously, the embedded path just pointed to the build directory, which would never work. Now, linking to this library will work if you specify the -Wl,-rpath,/path/to/lib option when linking an application against this library.
• Update Cantera.mak to add rpath flags when linking to Cantera or other libraries. This helps whenever these are shared libraries but not in standard system locations, for instance when linking against libraries in a conda installation.
• Fix an unrelated issue reported in the Users' Group (see https://groups.google.com/g/cantera-users/c/3PsUAFqeEhw) related to the fix applied for Install procedure tries to uninstall Cantera-python version installed by system pkg manager #1230 in Prevent aggressive pip uninstallation of existing package #1239.

Checklist

• The pull request includes a clear description of this code change
• Commit messages have short titles and reference relevant issues
• Build passes (scons build & scons test) and unit tests address code coverage
• Style & formatting of contributed code follows contributing guidelines
• The pull request is ready for review

[ischoegl]

Looks good to me.