Fix rpath handling to enable relocatable builds

This is needed for the Coin task that reuses host artifacts for target builds.

Specifically the rpaths that are to be embedded into $prefix/bin apps, into libraries, frameworks and plugins are different. They differ between OSes as well, e.g. $ORIGIN vs @loader_path or @executable_path.

This also needs to support the qmake configure equivalent of "-R ." where "." is a relative path relative to ${CMAKE_INSTALL_PREFIX}/${INSTALL_LIBDIR}.

Also on macOS we have some additional uikit vs non uikit checks for shallow bundles.

And we might need some of the "-rpath-link" magic on linux, but i'm not sure yet why that's added by qmake only to 2 tests and 1 plugin in a linux desktop configuration.

Obviously this doesn't affect Windows and static builds (I hope).