gettext: fix stdenv-darwin; remove Bison dependency (#59553)

Commit 12790167 broke building
nixpkgs.clangStdenv (stdenv-darwin) on macOS:

    nix-build -A clangStdenv .
    these derivations will be built:
      /nix/store/b3vyrfzarzyh51n7fj9wrpw12sq3765l-stdenv-darwin.drv
    building '/nix/store/b3vyrfzarzyh51n7fj9wrpw12sq3765l-stdenv-darwin.drv'...
    output '/nix/store/3agkxvkbhfpimnx5imr09q4rq9pxmjda-stdenv-darwin' is not allowed to refer to the following paths:
         /nix/store/jnzhx3hni0ds1wf49z8wfnxi3qdnkk6q-gnum4-1.4.18
           /nix/store/smdncnz5pkl5q39k4ldk8bh84p2hmj19-bison-3.3.2
    error: build of '/nix/store/b3vyrfzarzyh51n7fj9wrpw12sq3765l-stdenv-darwin.drv' failed

Remove gettext's build dependency on Bison to avoid this error, fixing
Nixpkgs on macOS.

To avoid rebuilding everything unnecessarily on Linux, only apply these
changes to macOS and keep the Bison dependency for Linux.