add --ninja argument to set ninja binary if not on system PATH (#59)

configure.py

@@ -102,6 +102,12 @@ parser.add_argument(
     type=Path,
     help="path to sjiswrap.exe (optional)",
 )
+parser.add_argument(
+    "--ninja",
+    metavar="BINARY",
+    type=Path,
+    help="path to ninja binary (optional)"
+)
 parser.add_argument(
     "--verbose",
     action="store_true",
@@ -141,6 +147,7 @@ config.compilers_path = args.compilers
 config.generate_map = args.map
 config.non_matching = args.non_matching
 config.sjiswrap_path = args.sjiswrap
+config.ninja_path = args.ninja
 config.progress = args.progress
 if not is_windows():
     config.wrapper = args.wrapper