Create a new capturing substitution for adding a rpath to a target
library. This is needed as Windows doesn't really support the concept
of a rpath. This also makes it possible to remove the parameter from
the command line on windows.
Thanks to @jrose for the hint about the substitution ordering, the new
substitution now works even inside the capture group. Replace the
remaining uses to the new macro.
The naming convention is different on Windows than on Unix-like
environments. In order to follow the convention we need to substitute
the prefix and the suffix. Take the opportunity to rename the
`target-dylib-extension` to the CMake-like variable
`target-shared-library-suffix` and introduce
`target-shared-library-prefix`. This helps linking the test suite
binaries on Windows.
