show default platform name.

ppremake/ppremake.cxx

@@ -83,7 +83,8 @@ usage() {
     "               files that would change.\n"
     "  -N           Verbose dry run: show the output of diff for the files\n"
     "               that would change (not supported in Win32-only version).\n\n"
-    "  -p platform  Build as if for the indicated platform name.\n"
+    "  -p platform  Build as if for the indicated platform name.  The default\n"
+    "               for this build is \"" << PLATFORM << "\".\n"
     "  -c config.pp Read the indicated user-level config.pp file after reading\n"
     "               the system config.pp file.  If this is omitted, the value\n"
     "               given in the environment variable PPREMAKE_CONFIG is used\n"