lib: Infer libc field of platform if not specified

This is especially useful when not cross compiling. It means we can
remove the `stdenv.isGlibc` predicate too.

Additionally, use this to simplify the logic to choose the
appropriate libiconv derivation.

lib/systems/default.nix

@@ -21,6 +21,12 @@ rec {
       config = parse.tripleFromSystem final.parsed;
       # Just a guess, based on `system`
       platform = platforms.selectBySystem final.system;
+      libc =
+        /**/ if final.isDarwin then "libSystem"
+        else if final.isMinGW  then "msvcrt"
+        else if final.isLinux  then "glibc"
+        # TODO(@Ericson2314) think more about other operating systems
+        else                        "native/impure";
     } // mapAttrs (n: v: v final.parsed) inspect.predicates
       // args;
   in final;