Add support for installing binaries with DT_RPATH

It's better to use DT_RPATH dynamic section of the elf binary to store
the paths of libraries to load instead of overwriting LD_LIBRARY_PATH
for the whole environment, causing surprises. This solution is much more
transparent and perfectly suitable for running contained installations
of wayland/wlroots/sway.

The code unsetting the LD_LIBRARY_PATH/LD_PRELOAD was also deleted as
it's a placebo security at best - we should trust the execution path
that leads us to running sway, and it's way too late to care about those
variables since we already started executing our compositor, thus we
would be compromised anyway.

swaymsg/meson.build

@@ -4,5 +4,6 @@ executable(
     include_directories: [sway_inc],
     dependencies: [jsonc, wlroots],
     link_with: [lib_sway_common],
+    install_rpath : rpathdir,
     install: true
 )