Drop UBUNTU_MENUPROXY=0 workaround wxWidgets 2.6 did not show menus correct on the global menu applet in Unity in 2010 ([Ubuntu bug #662007](https://launchpad.net/bugs/662077)). The environment variable `UBUNTU_MENUPROXY` was set as workaround.
Audacity 3.2.0 with wxWidgets 3.2 on Ubuntu 22.10 (kinetic) works fine under the Unity desktop. Therefore drop the old workaround.
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.
