[Build System: build-script] Allow users to enable distcc via the USE_DISTCC environment variable.

Author: Ross Bayer

utils/build-script

@@ -923,7 +923,8 @@ def main_preset():
         "--distcc",
         help="use distcc",
         action=argparse.actions.StoreTrueAction,
-        nargs=argparse.Nargs.OPTIONAL)
+        nargs=argparse.Nargs.OPTIONAL,
+        default=os.environ.get('USE_DISTCC') == '1')
     parser.add_argument(
         "--cmake-c-launcher",
         help="the absolute path to set CMAKE_C_COMPILER_LAUNCHER",

utils/build_swift/build_swift/driver_arguments.py

@@ -400,6 +400,7 @@ def create_argument_parser():
     option('--host-libtool', store_path(executable=True),
            help='the absolute path to libtool. Default is auto detected.')
     option('--distcc', toggle_true,
+           default=os.environ.get('USE_DISTCC') == '1',
            help='use distcc in pump mode')
     option('--enable-asan', toggle_true,
            help='enable Address Sanitizer')