Meson is a delicate package. It needs some patches to properly work in Nixpkgs
and many packages have it as a dependency, from many small applications to
systemd (yay, mass rebuilds in sight!). Updating it is not a trivial and
harmless task.
Therefore, I took the paranoid way and encapsulate the current version in a
subdir, instead of the regular write-over procedure. This way, at the best we
can just remove the whole dir in the future, and at the worse we just maintain
duplicated code.
Meson allows projects to set `build_rpath` property, containing paths
that will be added during build but will be removed when installing.
When Meson removes build_rpath from `DT_RUNPATH` entry, it just writes
the shorter ␀-terminated new rpath over the old one to reduce
the risk of potentially breaking the ELF files
(when the linker does string de-duplication or something).
But this can cause much bigger problem for Nix, as it can produce
cut-in-half-by-␀ store path references.
For example, in systemd’s libudev, it was removing three `$ORIGIN`-relative paths from
$ORIGIN/../libsystemd:$ORIGIN/../basic:$ORIGIN/../shared:…␀
resulting in the following `DT_RUNPATH` entry:
…␀store/v589pqjhvxrj73g3r0xb41yr84z5pwb7-gcc-9.3.0-lib/lib␀
We previously handled this in `fix-rpath.patch` but the method we prevent
Meson from removing paths added to rpath through `NIX_LDFLAGS` was changed
during 0.55.0 update and I forgot about this second purpose of the patch.
Let’s re-add this clearing code, as it worked without issues for a long time.
