flatc now creates the output directory if it doesn't exist yet.

Also cleaned up the general mkdir functionality.

Tested: on OS X.

Tested that a command line like: ./flatc -j -o foo/bar/baz tests/monster_test.fbs
generates files in foo/bar/baz/MyGame/Example/*.java which previously didn't exist.

Windows code was previously tested but has been moved, so needs to be tested again.

Change-Id: Iee943121b3be4f92961a3ec94c2cb578165f114a

src/flatc.cpp

@@ -180,6 +180,7 @@ int main(int argc, const char *argv[]) {
 
       for (size_t i = 0; i < num_generators; ++i) {
         if (generator_enabled[i]) {
+          flatbuffers::EnsureDirExists(output_path);
           if (!generators[i].generate(parser, output_path, filebase, opts)) {
             Error((std::string("Unable to generate ") +
                    generators[i].name +