meson: fix rpath fixup

In common distributions, RPATH is only needed for internal libraries so
meson removes everything else. With Nix, the locations of libraries
are not as predictable, therefore we need to keep them in the RPATH. [1]

Previously we have just kept the RPATH produced by the linker, patching
meson not to remove it. This deprived us of potentially replacing it
with install_rpath provided by project so we had to re-add it manually,
and also introduced a vulnerability of keeping build paths in RPATH.

This commit restores the clean-up but modifies it so the items starting
with /nix/store are retained.

This should be relatively safe since the store is immutable, however,
there might be some unwanted retainment of build_rpath [2] if it contains
paths from Nix store.

[1]: https://github.com/NixOS/nixpkgs/issues/31222#issuecomment-365811634
[2]: http://mesonbuild.com/Release-notes-for-0-42-0.html#added-build_rpath-keyword-argument