1. First of all, this doesn't work in nixpkgs. Per [1], gcc ignores `-L`
for purposes of `--print-file-dirs`, which breaks horribly on linux. But
if we don't pass extra dirs, meosn first just tries `-l...`, which does
work.
2. Even if it did work, `libdir` means where we are installing libs, not
where libs are expected to be found. Those are not necessarily the
same (again, nixpkgs), and even when they are and non-standard, it is
better to use DESTDIR or have a modified toolchain.
[1]: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87758
(cherry picked from commit a142164e746644e20f66908c156ca913bef4664f)