getopt: fix build on clang 16

Normally, when getopt builds on a non-glibc platform, it uses C library
headers without including them. That’s fine unless your compiler is
clang 16, which is strict about using functions without prototypes.
Fortunately, Darwin’s headers are compatible enough as are Musl’s, so
defining `__GNU_LIBRARY__` can be done unconditionally.

pkgs/tools/misc/getopt/default.nix

@@ -8,6 +8,10 @@ stdenv.mkDerivation rec {
     sha256 = "1zn5kp8ar853rin0ay2j3p17blxy16agpp8wi8wfg4x98b31vgyh";
   };
 
+  # This should be fine on Linux and Darwin. Clang 16 requires it because otherwise getopt will
+  # attempt to use C library functions without declaring them, which is raised as an error.
+  env.NIX_CFLAGS_COMPILE = "-D__GNU_LIBRARY__";
+
   makeFlags = [
     "WITHOUT_GETTEXT=1"
     "LIBCGETOPT=0"