Support OTP-21's unicode-only constraints

Functions start being available in OTP-20, so to get rid of warnings, a
conditional compilation flag is set there.

The cspan function has no direct equivalent, but can be worked around.
Likely heavier work would be required around its usage where strings are
not handled as lists, but only through 'string' functions. However this
will still cover most cases and get rid of compile-time warnings for
now.

rebar.config

@@ -11,5 +11,7 @@
             warn_export_vars,
             warn_exported_vars,
             warn_missing_spec,
-            warn_untyped_record, debug_info]}.
+            warn_untyped_record, debug_info,
+            {platform_define, "^2", unicode_str}
+]}.
 {xref_checks, [undefined_function_calls]}.

rebar.lock

@@ -0,0 +1 @@
+[].

src/getopt.erl

@@ -442,7 +442,7 @@ to_type(integer, Arg) ->
 to_type(float, Arg) ->
     list_to_float(Arg);
 to_type(boolean, Arg) ->
-    LowerArg = string:to_lower(Arg),
+    LowerArg = lowercase(Arg),
     case is_arg_true(LowerArg) of
         true ->
             true;
@@ -500,7 +500,7 @@ is_implicit_arg(_Type, _Arg) ->
 
 -spec is_boolean_arg(string()) -> boolean().
 is_boolean_arg(Arg) ->
-    LowerArg = string:to_lower(Arg),
+    LowerArg = lowercase(Arg),
     is_arg_true(LowerArg) orelse is_arg_false(LowerArg).
 
 
@@ -619,7 +619,7 @@ usage_cmd_line(ProgramName, OptSpecList, CmdLineTail) ->
 %%      already wrapped according to the maximum MaxLineLength.
 -spec usage_cmd_line_options(MaxLineLength :: non_neg_integer(), [option_spec()], CmdLineTail :: string()) -> iolist().
 usage_cmd_line_options(MaxLineLength, OptSpecList, CmdLineTail) ->
-    usage_cmd_line_options(MaxLineLength, OptSpecList ++ string:tokens(CmdLineTail, " "), [], 0, []).
+    usage_cmd_line_options(MaxLineLength, OptSpecList ++ lexemes(CmdLineTail, " "), [], 0, []).
 
 usage_cmd_line_options(MaxLineLength, [OptSpec | Tail], LineAcc, LineAccLength, Acc) ->
     Option = [$\s | lists:flatten(usage_cmd_line_option(OptSpec))],
@@ -790,7 +790,7 @@ wrap_text_line(Length, [_ | _] = Help, Acc, Count, CurrentLineAcc) ->
     %% Look for the first whitespace character in the current (reversed) line
     %% buffer to get a wrapped line. If there is no whitespace just cut the
     %% line at the position corresponding to the maximum length.
-    {NextLineAcc, WrappedLine} = case string:cspan(CurrentLineAcc, " \t") of
+    {NextLineAcc, WrappedLine} = case cspan(CurrentLineAcc, " \t") of
                                      WhitespacePos when WhitespacePos < Count ->
                                          lists:split(WhitespacePos, CurrentLineAcc);
                                      _ ->
@@ -933,3 +933,15 @@ to_string(Atom) when is_atom(Atom) ->
     atom_to_list(Atom);
 to_string(Value) ->
     io_lib:format("~p", [Value]).
+
+%% OTP-20/21 conversion to unicode string module
+-ifdef(unicode_str).
+lowercase(Str) -> string:lowercase(Str).
+lexemes(Str, Separators) -> string:lexemes(Str, Separators).
+cspan(Str, Chars) -> length(element(1,string:take(Str, Chars, true))).
+-else.
+lowercase(Str) -> string:to_lower(Str).
+lexemes(Str, Separators) -> string:tokens(Str, Separators).
+cspan(Str, Chars) -> string:cspan(Str, Chars).
+-endif.
+