meson: Add -Wl,-rpath,${libdir} on macOS
We made the gstreamer installation prefix relocatable by picking up plugins relative to the location of libgstreamer-1.0.dylib, similar to how it's done for Windows:
This had a lot of side-effects:
#1051 (closed) cerbero#363 (closed) cerbero#371 (closed) cerbero#362 (closed)
A partial fix for the cerbero side of these was:
However, this relied on the consumers knowing that they need to add
LC_RPATH
entries to the libdir of the prefix. This is done
automatically by build systems like Meson, but not by others, such as
Autotools, CMake, Cargo, XCode, etc. For those, we need to add the
RPATH entries to the gstreamer-1.0.pc file.
This also has the side-effect of fixing the loading of gstreamer rust plugins on macOS:
Fixes #1159 (closed)
Fixes #1149 (closed)