jemalloc-sys fails to compile with clang 15

[delan]

Reported by @mukilan with NixOS 23.05 in #31055:

error: failed to run custom build command for `jemalloc-sys v0.5.4+5.3.0-patched`
 
Caused by:
  process didn't exit successfully: `/home/mukilan/dev/servo/target/debug/build/jemalloc-sys-128f6f2e7894a2a7/build-script-build` (exit status: 101)
[...]
  src/malloc_io.c:107:9: error: incompatible pointer to integer conversion returning 'char *' from a function with result type 'int' [-Wint-conversion]
          return strerror_r(err, buf, buflen);
                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
  1 warning generated.
  1 warning and 1 error generated.
  In file included from make: *** [Makefile:478: src/malloc_io.sym.o] Error 1
  make: *** Waiting for unfinished jobs....
[...]
  thread 'main' panicked at /home/mukilan/.cargo/registry/src/index.crates.io-6f17d22bba15001f/jemalloc-sys-0.5.4+5.3.0-patched/build.rs:351:9:
  command did not execute successfully: cd "/home/mukilan/dev/servo/target/debug/build/jemalloc-sys-0a3234e2aba5603a/out/build" && "make" "-j" "24"
  expected success, got: exit status: 2
  note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
warning: build failed, waiting for other jobs to finish...

If your distro lets you install older versions of clang with a package like clang-14, you can tell Servo to use it with:

export CC=/usr/bin/clang-14
export CXX=/usr/bin/clang++-14

Alternatively you can try our new Nix-based dev environment, which should now work on any Linux distro. Nix is a package manager with some unusual benefits. Servo can use Nix to find the correct versions of all of its compilers and build dependencies without needing you to install them or run mach bootstrap. All you need to do is install Nix, and export MACH_USE_NIX= to your environment. See the wiki for more details!