Fix rpath for private libraries on Linux

Fix rpath for private libraries on Linux

Installing audacity on Linux will produce private libraries that have
`RUNPATH` set to the build directory instead of the installation
directory.

The root cause is that the library directory is copied to the
installation directory without touching the libraries. The cmake wiki
says in RPATH handling caveats [1]: "Since install-side RPATH tweaking
is an operation that is done by target-specific installation handling,
any target that should have its install RPATH changed (e.g. to
`CMAKE_INSTALL_RPATH`) needs to end up in the installation via an
`install(TARGETS ...)` signature and not via directory-based copying."

So replace `install(DIRECTORY ...)` by individual `install(TARGETS ...)`
for the libraries and modules. Then cmake will replace the `RUNPATH` to
`$ORIGIN/../lib/audacity`, which is still incorrect. Therefore set
`INSTALL_RPATH` explicitly.

Fixes: https://github.com/audacity/audacity/issues/3289
Forwarded: https://github.com/audacity/audacity/pull/3671
[1] https://gitlab.kitware.com/cmake/community/-/wikis/doc/cmake/RPATH-handling#caveats
Signed-off-by: Benjamin Drung <benjamin.drung@canonical.com>
Gbp-Pq: Name Fix-rpath-for-private-libraries-on-Linux.patch