Allow Ginkgo to Use the `--prefix` Option

[MarcelKoch]

This PR allows Ginkgo to use the --prefix option of cmake --install. Before, the path set by --prefix would just be ignored and the CMAKE_INSTALL_PREFIX set at configure time would be used.

In principle, this is a minor change, except for the PKG config file. Getting this to work is quite cumbersome, but now it should work as expected.

Changes to the pkg-config file:

• The prefix now uses the path specified by --prefix (if present)
• libdir and includedir are now relative to prefix
• The -L and -I paths for ginkgo also use paths relative to prefix

[upsj]

@MarcelKoch can you maybe list the behavioral changes in the pkg-config flag generation so we can check them against the code?

[sonarcloud[bot]]

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