This enables us to generate pkg-config with proper paths if the
installation prefix is decided at install time, e.g. with
```
cmake --install build-dir --prefix /path/to/somewhere
```
It also means that we can use CMake's generator expression to get
the _real_ final name of the libraries, e.g. including debug postfix.
Closes#2979
This describes the reality better, as it also links in the rest
of Catch2.
The on-disk name of the static library remains just `Catch2Main`,
as that is what it is -- single main function -- and on-disk artifacts
cannot describe link dependencies.
