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.
The old `CC=.. CXX= .. meson ...` env var hack I removed in
3c00ca03a2 had a side effect of ensuring
that Meson always had access to a native C compiler, which unforunately
it expects in most cases. Thankfully, that will be fixed soon.
The cross file is added in the `mkDerivation`. It isn't nice putting
build tool-specific stuff here, but our current architecture gives us
little alternative.
See comment in code and the PR it references,
https://github.com/mesonbuild/meson/pull/6827, for details.
We can remove entries from the cross file because they will be gotten
from env vars now.
As documented, it should be `aarch64` for AArch64.
* https://mesonbuild.com/Reference-tables.html#cpu-families
```
$ nix eval '((import <nixpkgs> {}).pkgsCross.aarch64-multiplatform.stdenv.targetPlatform.parsed.cpu.family)'
"arm"
```
The lookup table will ensure that, at any point, meson does not pick the
wrong family.
I *want* cross-specific overrides to be verbose, so I rather not have
this shorthand. This makes the syntactic overhead more proportional to
the maintainence cost. Hopefully this pushes people towards fewer
conditionals and more abstractions.
