Add support for arch specific port_sources

On one project I have a need to specify port_sources on R14 only
and on another different project port_sources for Darwin and Linux.
To this end add support to handle tuples of the form
{ArchRegex, PortSource} in the port_sources list, eg:

    {port_sources, [{"R14", ["c_src/*.c"]}]}.

src/rebar_port_compiler.erl

@@ -37,7 +37,11 @@
 
 %% Supported configuration variables:
 %%
-%% * port_sources - Erlang list of files and/or wildcard strings to be compiled
+%% * port_sources - Erlang list of files and/or wildcard strings to be
+%%                  compiled. Platform specific sources can be specified
+%%                  by enclosing a string in a tuple of the form
+%%                  {Regex, String} wherein Regex is a regular expression
+%%                  that is checked against the system architecture.
 %%
 %% * so_specs  - Erlang list of tuples of the form {"priv/so_name.so", ["c_src/object_file_name.o"]} useful for
 %%               building multiple *.so files.
@@ -136,6 +140,14 @@ clean(Config, AppFile) ->
 
 expand_sources([], Acc) ->
     Acc;
+expand_sources([{ArchRegex, Spec} | Rest], Acc) ->
+    case rebar_utils:is_arch(ArchRegex) of
+        true ->
+            Acc2 = filelib:wildcard(Spec) ++ Acc,
+            expand_sources(Rest, Acc2);
+        false ->
+            expand_sources(Rest, Acc)
+    end;
 expand_sources([Spec | Rest], Acc) ->
     Acc2 = filelib:wildcard(Spec) ++ Acc,
     expand_sources(Rest, Acc2).